drm/i915/crtc: rename intel_get_crtc_for_plane() to intel_crtc_for_plane()

The "get" in the name implies reference counting, remove it. This also
makes the function conform to naming style.

Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/50359b38c0eabe262ff31c9ec35c97aa5dfb7fef.1638366969.git.jani.nikula@intel.com