drm/amdgpu: add gfx shadow callback

To provide IP specific shadow sizes.  UMDs will use
this to query the kernel driver for the size of the
shadow buffers.

v2: make callback return an int (Alex)
v3: drop GDS (Alex)

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