drm/amdgpu: disable rom clock gating support for APUs

ROM clock gating enable/disable is not supported
on APU platform. (i.e. CGTT_ROM_CLK_CTRL0 register
is not availabe on APU). SMUIO callbacks will check
APU flag before enable/disable rom clock gating, and
skip the programming. Accordingly, query clock gating
status through CGTT_ROM_CLK_CTRL0 also doesn't support
on APU platform.

The change applies to RAVEN/RAVEN2/PICASSO/RENOIR.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>