drm/amdgpu/vcn: drop gfxoff control for VCN2+

Drop disabling of gfxoff during VCN use.  This allows gfxoff
to kick in and potentially save power if the user is not using
gfx for color space conversion or scaling.

VCN1.0 had a bug which prevented it from working properly with
gfxoff, so we disabled it while using VCN.  That said, most apps
today use gfx for scaling and color space conversion rather than
overlay planes so it was generally in use anyway. This was fixed
on VCN2+, but since we mostly use gfx for color space conversion
and scaling and rapidly powering up/down gfx can negate the
advantages of gfxoff, we left gfxoff disabled. As more
applications use overlay planes for color space conversion
and scaling, this starts to be a win, so go ahead and leave
gfxoff enabled.

Note that VCN1.0 uses vcn_v1_0_idle_work_handler() and
vcn_v1_0_ring_begin_use() so they are not affected by this
patch.

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