libdrm: clean up dependencies

[geimer]

Both libpthread-stubs and libpciaccess are included as components in X11, so we can safely remove them here.

[geimer]

Test report by @geimer
SUCCESS
Build succeeded for 1 out of 1 (1 easyconfigs in this PR)
swat - Linux debian 7.11, Intel(R) Xeon(R) CPU X5550 @ 2.67GHz, Python 2.7.3
See https://gist.github.com/08eb1e9f313bf5def0936d29015f1a92 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in this PR)
node2605.swalot.os - Linux centos linux 7.3.1611, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz, Python 2.7.5
See https://gist.github.com/d8f3e02718d402cef51f6fca292ad669 for a full test report.

[boegel]

Test report by @boegel
SUCCESS
Build succeeded for 2 out of 2 (2 easyconfigs in this PR)
node2086.delcatty.os - Linux centos linux 7.3.1611, Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz, Python 2.7.5
See https://gist.github.com/08595640b9d2ffafeb280f35c9d6d522 for a full test report.

[boegel]

lgtm

[boegel]

Going in, thanks @geimer!