drm/i915: Fix the docs for intel_set_cdclk_post_plane_update()

drm/i915: Fix the docs for intel_set_cdclk_post_plane_update()

s/before/after/ again after accidentally changing it the
other way in commit 5604e9ceaed5 ("drm/i915: Simplify
intel_set_cdclk_{pre,post}_plane_update() calling convention")

Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204154803.25403-1-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>