drm/i915/display: Fix glitches when moving cursor with PSR2 selective fetch enabled

Legacy cursor APIs are handled by intel_legacy_cursor_update(), that
calls drm_atomic_helper_update_plane() when going through the
slow/atomic path to update cursor, what was the case for PSR2
selective fetch.

drm_atomic_helper_update_plane() sets
drm_atomic_state->legacy_cursor_update to true when updating the
cursor plane, to allow several cursor updates to happen within the
same frame, as userspace does that.
If drivers waited for a vblank increment at the end of every cursor
movement that would cause a visible lag in the cursor.

But this optimization do not properly work with PSR2 selective fetch
dirt area calculation, for example if within a single frame the cursor
had 3 moves the final dirt area programmed to PSR2_MAN_TRK_CTL would
be based in the second movement as old state and third movement as new
state, not updating the area where cursor was in the first state.

So here switching back to the fast path approach in
intel_legacy_cursor_update() and handling cursor movements as
frontbuffer rendering(psr_force_hw_tracking_exit()), that is not the
most optimal for power-savings but is the solution that we have until
mailbox style updates is implemented.

Also removing the cursor workaround as not it is properly undestand
the issue and is know that it will never cover all the cases.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930001409.254817-5-jose.souza@intel.com