drm/amd/display: check attr flag before set cursor degamma on DCN3+

Don't set predefined degamma curve to cursor plane if the cursor
attribute flag is not set. Applying a degamma curve to the cursor by
default breaks userspace expectation. Checking the flag before
performing any color transformation prevents too dark cursor gamma in
DCN3+ on many Linux desktop environment (KDE Plasma, GNOME,
wlroots-based, etc.) as reported at:
- https://gitlab.freedesktop.org/drm/amd/-/issues/1513

This is the same approach followed by DCN2 drivers where the issue is
not present.

Fixes: 03f54d7d ("drm/amd/display: Add DCN3 DPP")
Link: https://gitlab.freedesktop.org/drm/amd/-/issues/1513


Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Tested-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org