drm/amdgpu: validate user GEM object size

When creating frame buffer, userspace may request to attach to a
previously allocated GEM object that is smaller than what GPU
requires. Validation must be done to prevent out-of-bound DMA,
otherwise it could be exploited to reveal sensitive data.

This fix is not done in a common code path because individual
driver might have different requirement.

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>