drm/amdgpu: Drop drm/ prefix for including drm.h in amdgpu_drm.h

This allows amdgpu_drm.h to be reused verbatim in libdrm.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>