drm/i915: Check crtc_state->wm.need_postvbl_update before grabbing wm.mutex

wm.mutex does not protect the crtc state so no point in grabbing it
to check crtc_state->wm.need_postvbl_update.

Also do a bit of s/intel_crtc/crtc/ while at it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190701160550.24205-1-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>