drm/i915: Factor out function to clear pipe update flags

Factor out a helper to clear the pipe update flags, used by a follow-up
patch to modeset an MST topology.

v2:
- Move the intel_crtc_needs_modeset() check to the callers. (Ville)
v3 (Ville):
- Rename clear_pipe_update_flags_on_modeset_crtc() to
  intel_crtc_flag_modeset().
- Also set crtc_state->uapi.mode_changed in the function.
- Leave out the unrelated change to use
  intel_modeset_pipes_in_mask_early().

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107001505.3370108-7-imre.deak@intel.com