mesa: allow mesa (gbm) to compile without backend

mesa: allow mesa (gbm) to compile without backend

Commit introduces a patch that allows for gbm to
be built with an empty backend. There are situation
where mesa-gl is the preferred provider for virtual/libgbm,
virtual/libgl, virtual/mesa, etc... But the x11 DISTRO_FEATURE
isn't included this leads to build errors such as

| /../../../ld: src/gbm/libgbm.so.1.0.0.p/main_backend.c.o: in function
`find_backend':
| backend.c:(.text.find_backend+0xa4): undefined reference to
`gbm_dri_backend'
| /../../../ld:
src/gbm/libgbm.so.1.0.0.p/main_backend.c.o:(.data.rel.ro.builtin_backends+0x4):
undefined reference to `gbm_dri_backend'
| collect2: error: ld returned 1 exit status

Add patch to bypass compilation issue by excluding gbm dri backend.

https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20447
https://gitlab.freedesktop.org/mesa/mesa/-/commit/842ca284650f066e58706741a7d22d67b5088e60

Commit also includes the use of TARGET_CFLAGS variable in mesa-gl recipe
to fix bellow compilation bug.

/../recipe-sysroot/usr/include/xf86drm.h:40:10: fatal error: drm.h: No such file or directory
|    40 | #include <drm.h>

Signed-off-by: Vincent Davis Jr <vince@underview.tech>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>