drm/amd/display: Fix mem_type change detection for async flips

drm/amd/display: Fix mem_type change detection for async flips

[Why]
amdgpu_dm_crtc_mem_type_changed() fetches the "old" and "new" plane state
with two drm_atomic_get_plane_state() calls, which both return the new
state. It compares a state against itself, so it never detects a mem_type
change and never rejects the async flip.

On DCN 3.0.1, this shows up as intermittent corruption when a single DCC
plane is scanned out with immediate flips under gamescope and its buffer
moves between the VRAM carveout and GTT.

[How]
Use drm_atomic_get_old_plane_state() and drm_atomic_get_new_plane_state()
to compare the actual old and new states. These return NULL rather than
an error pointer for a plane that is not part of the commit, so the
IS_ERR() check becomes a NULL check that skips those planes, such as an
unmodified cursor still in the CRTC's plane_mask.

Fixes: 4caacd1671b7 ("drm/amd/display: Do not elevate mem_type change to full update")
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Matthew Schwartz <matthew.schwartz@linux.dev>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 13158e5dbd896281f3e9982b5437cffa5fd621b2)