disable generating of man pages in recent libdrm easyconfigs to avoid installation failure if docbook-xsl is not installed

[boegel]

(created using eb --new-pr)

This avoids a failing installation when docbook-xsl is not installed in the OS:

I/O I/O error : error : Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
I/O warning: warning: error : failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
Attempt to load network entity http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl"
cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
make[2]: *** [Makefile:626: drmHandleEvent.3] Error 4

[lexming]

Test report by @lexming
SUCCESS
Build succeeded for 5 out of 5 (5 easyconfigs in this PR)
node315.hydra.os - Linux centos linux 7.7.1908, x86_64, Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz, Python 2.7.5
See https://gist.github.com/664b366ecfed2dd52d6356ce1b1d2717 for a full test report.

[lexming]

Test report by @lexming
SUCCESS
Build succeeded for 5 out of 5 (5 easyconfigs in this PR)
node116.hydra.os - Linux centos linux 7.7.1908, x86_64, Intel(R) Xeon(R) CPU E5-2680 v2 @ 2.80GHz, Python 2.7.5
See https://gist.github.com/847e8bf4e3ec338c9092edd364f0db0b for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 5 out of 5 (5 easyconfigs in this PR)
node3170.skitty.os - Linux centos linux 7.8.2003, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz (skylake_avx512), Python 3.6.8
See https://gist.github.com/4f73f7543d66a08f755639528fcb7c12 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 5 out of 5 (5 easyconfigs in this PR)
node2510.golett.os - Linux centos linux 7.8.2003, x86_64, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz (haswell), Python 2.7.5
See https://gist.github.com/1ea243d34739a4286e6cedb7c8378f18 for a full test report.

[lexming]

LGTM

[lexming]

Going in, thanks @boegel !