drm/i915: Fixup kerneldoc for intel_pm.c

drm/i915: Fixup kerneldoc for intel_pm.c

drivers/gpu/drm/i915/intel_pm.c:750: warning: Function parameter or member 'fifo_size' not described in 'intel_calculate_wm'
drivers/gpu/drm/i915/intel_pm.c:5900: warning: Function parameter or member 'crtc' not described in 'intel_update_watermarks'

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214140303.1561-1-chris@chris-wilson.co.uk
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>