drm/amd/display: remove usage of legacy_cursor_update

Currently the atomic check code uses legacy_cursor_update
to differnetiate if the cursor plane is being requested by
the user, which is not required as we shall be updating
plane only if modeset is requested/required.

Have tested cursor plane and underlay get updated seamlessly,
without any lag or frame drops.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>