drm/vkms: Convert vkms_crtc_atomic_check() to use new plane state

drm/vkms: Convert vkms_crtc_atomic_check() to use new plane state

The vkms_crtc_atomic_check() function calls the deprecated
drm_atomic_get_existing_plane_state() helper for its internal mode
checking logic.

During atomic_check, the existing state is the new state and
drm_atomic_get_existing_plane_state() can thus be replaced by
drm_atomic_get_new_plane_state().

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-6-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>