drm/amdgpu: Use correct mask in dce_v8_0_afmt_setmode() and fix comment typos.

We were using the same mask twice. Looking at radeon, it seems
we should be using HDMI_AVI_INFO_CONT instead as the second mask.

Being there, fix typos in comments and improved readability.

I haven't looked at other DCEs, the mask may also be wrong for them.

Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>