drm/amd/display: enable cursor degamma for DCN3+ DRM legacy gamma

For DRM legacy gamma, AMD display manager applies implicit sRGB degamma
using a pre-defined sRGB transfer function. It works fine for DCN2
family where degamma ROM and custom curves go to the same color block.
But, on DCN3+, degamma is split into two blocks: degamma ROM for
pre-defined TFs and `gamma correction` for user/custom curves and
degamma ROM settings doesn't apply to cursor plane. To get DRM legacy
gamma working as expected, enable cursor degamma ROM for implict sRGB
degamma on HW with this configuration.

Cc: stable@vger.kernel.org
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2803


Fixes: 96b020e2 ("drm/amd/display: check attr flag before set cursor degamma on DCN3+")
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>