drm/amdgpu: Use drm_dbg_kms for reporting failure to get a GEM FB

drm_err meant broken user space could spam dmesg.

Fixes: f258907f

 "drm/amdgpu: Verify bo size can fit framebuffer size on init."
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>