drm: Fix documentation generation for DP_DPCD_QUIRK_NO_PSR

The DP_DPCD_QUIRK_NO_PSR comment is missing colon causing this
warning when generating kernel documentation.

./include/drm/drm_dp_helper.h:1374: warning: Incorrect use of kernel-doc format:          * @DP_DPCD_QUIRK_NO_PSR

Fixes: 7c5c641a (drm/i915: Disable PSR in Apple panels)
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181205184850.13218-1-jose.souza@intel.com