drm/i915: Fix possible NULL pointer dereferences in i9xx_update_wm()

Check return pointer from intel_crtc_for_plane() before dereferencing
it, as it can be NULL.

v2: Moved the NULL check into intel_crtc_active().

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Caz Yokoyama <caz.yokoyama@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211217160255.1300348-1-harish.chegondi@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -877,7 +877,7 @@ static bool intel_crtc_active(struct intel_crtc *crtc)
 	 * crtc->state->active once we have proper CRTC states wired up
 	 * for atomic.
 	 */
-	return crtc->active && crtc->base.primary->state->fb &&
+	return crtc && crtc->active && crtc->base.primary->state->fb &&
 		crtc->config->hw.adjusted_mode.crtc_clock;
 }