drm/amdgpu: validate user pitch alignment

Userspace may request pitch alignment that is not supported by GPU.
Some requests 32, but GPU ignores it and uses default 64 when cpp is
4. If GEM object is allocated based on the smaller alignment, GPU
DMA will go out of bound.

Cc: stable@vger.kernel.org # v4.2+
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>