mesa: enable only freedreno for hammerhead

* we don't need other drivers enabled by default:
  GALLIUMDRIVERS="swrast,r300,svga,nouveau,virgl,freedreno"
  GALLIUMDRIVERS_LLVM="r300,svga,nouveau"

        Gallium drivers: swrast r300 svga nouveau virgl freedreno
        Gallium st:      mesa

  and building svga driver for hammerhead fails with:
| ../../../../../mesa-18.3.4/src/gallium/drivers/svga/svga_msg.c: In function 'svga_host_log':
| ../../../../../mesa-18.3.4/src/gallium/drivers/svga/svga_msg.c:86:4: error: impossible constraint in 'asm'
|  ({                                         \
|  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|     __asm__ volatile ("inl %%dx, %%eax;" :  \
|     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "=a"(ax),                            \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "=b"(bx),                            \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "=c"(cx),                            \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "=d"(dx),                            \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "=S"(si),                            \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "=D"(di) :                           \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "a"(magic),                          \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "b"(in_bx),                          \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "c"(cmd),                            \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "d"(port_num),                       \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "S"(in_si),                          \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "D"(in_di) :                         \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|        "memory");                           \
|        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|  })
|  ~~

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>

meta-luneui/recipes-graphics/mesa/mesa_%.bbappend

@@ -4,4 +4,5 @@ PACKAGECONFIG_append_class-target = " gallium gallium-llvm"
 PACKAGECONFIG_append_class-target = " gbm"
 
 # Enable freedreno driver
-GALLIUMDRIVERS_append_hammerhead = ",freedreno"
+GALLIUMDRIVERS_hammerhead = "freedreno"
+GALLIUMDRIVERS_LLVM_hammerhead = ""