drm/amdgpu: add get_gfx_shadow_info callback for gfx11

Used to get the size and alignment requirements for
the gfx shadow buffer for preemption.

v2: use FW version check to determine whether to
    return a valid size here
    return an error if not supported (Alex)
v3: drop GDS (Alex)
v4: make amdgpu_gfx_shadow_info mandatory (Alex)

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>