drm/amd/display: remove need of modeset flag for overlay planes (V2)

drm/amd/display: remove need of modeset flag for overlay planes (V2)

This patch is in continuation to the
"843e3c7 drm/amd/display: defer modeset check in dm_update_planes_state"
where we started to eliminate the dependency on
DRM_MODE_ATOMIC_ALLOW_MODESET to be set by the user space,
which as such is not mandatory.

After deferring, this patch eliminates the dependency on the flag
for overlay planes.

This has to be done in stages as its a pretty complex and requires thorough
testing before we free primary planes as well from dependency on modeset
flag.

V2: Simplified the plane type check.

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