drm/i915: Introduce intel_crtc_needs_fastset()

Replace the somewhat obscure crtc_state.update_pipe checks
with a more descriptive thing. Also nicely matches the
intel_crtc_needs_modeset() counterpart for full modesets.

v2: Handle one more case in the fbc code

Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221021162442.27283-2-ville.syrjala@linux.intel.com