drm/i915: Drop redunant null check from intel_get_frame_time_us()

intel_get_frame_time_us() is never called with a NULL crtc_state so
drop the redundant check.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231207193441.20206-7-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>