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