drm/amd/amdgpu: enable GUI idle INT after enabling CGCG

GUI idle interrupts should be enabled only after we
have enabled coarse grain clock gating (CGCG). This
prevents GFX engine generating idle interrupt even
though CGCG is not completely enabled.

Most of the time this goes un-noticed, but on some
Stoney ASICs this results in GFX engine hang after
system resumes from suspend. The issue is not
particular to Stoney though and could have occured
on any ASIC. The patch fixes this issue.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reported-by: Sunil Uttarwar <Sunil.Uttarwar1@amd.com>
Signed-off-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org