radeon: hide pointless #warning when compile testing

commit c02216acf4177c4411d33735c81cad687790fa59 upstream.

In randconfig testing, we sometimes get this warning:

drivers/gpu/drm/radeon/radeon_object.c: In function 'radeon_bo_create':
drivers/gpu/drm/radeon/radeon_object.c:242:2: error: #warning Please enable CONFIG_MTRR and CONFIG_X86_PAT for better performance thanks to write-combining [-Werror=cpp]
 #warning Please enable CONFIG_MTRR and CONFIG_X86_PAT for better performance \

This is rather annoying since almost all other code produces no build-time
output unless we have found a real bug. We already fixed this in the
amdgpu driver in commit 31bb90f1 ("drm/amdgpu: shut up #warning for
compile testing") by adding a CONFIG_COMPILE_TEST check last year and
agreed to do the same here, but both Michel and I then forgot about it
until I came across the issue again now.

For stable kernels, as this is one of very few remaining randconfig
warnings in 4.14.

Cc: stable@vger.kernel.org
Link: https://patchwork.kernel.org/patch/9550009/


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>