drm/atomic: allow no-op FB_ID updates for async flips

User-space is allowed to submit any property in an async flip as
long as the value doesn't change. However we missed one case:
as things stand, the kernel rejects no-op FB_ID changes on
non-primary planes. Fix this by changing the conditional and
skipping drm_atomic_check_prop_changes() only for FB_ID on the
primary plane (instead of skipping for FB_ID on any plane).

Fixes: 0e26cc72 ("drm: Refuse to async flip with atomic prop changes")
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Tested-by: Xaver Hugl <xaver.hugl@kde.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Michel Dänzer <michel.daenzer@mailbox.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240731191014.878320-1-contact@emersion.fr