drm/atomic: Convert drm_atomic_get_crtc_state() to use new connector state

drm/atomic: Convert drm_atomic_get_crtc_state() to use new connector state

The drm_atomic_get_crtc_state() function calls the deprecated
drm_atomic_get_existing_crtc_state() helper to get find if a crtc state
had already been allocated and was part of the given drm_atomic_state.

At the point in time where drm_atomic_get_crtc_state() can be
called (ie, during atomic_check), the existing state is the new state
and drm_atomic_get_existing_crtc_state() can thus be replaced by
drm_atomic_get_new_crtc_state().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20250930-drm-no-more-existing-state-v5-10-eeb9e1287907@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>