drm: Fix docbook warnings in hdr metadata helper structures

Fixes the following warnings:
./include/drm/drm_mode_config.h:841: warning: Incorrect use of
kernel-doc format:          * hdr_output_metadata_property: Connector
property containing hdr
./include/drm/drm_mode_config.h:918: warning: Function parameter or member 'hdr_output_metadata_property' not described in 'drm_mode_config'
./include/drm/drm_connector.h:1251: warning: Function parameter or member 'hdr_output_metadata' not described in 'drm_connector'
./include/drm/drm_connector.h:1251: warning: Function parameter or member 'hdr_sink_metadata' not described in 'drm_connector'

Also adds some property documentation for HDR Metadata Connector
Property in connector property create function.

v2: Fixed Sean Paul's review comments.

v3: Fixed Daniel Vetter's review comments, added the UAPI structure
definition section in kernel docs.

v4: Fixed Daniel Vetter's review comments.

v5: Added structure member references as per Daniel's suggestion.

Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Ville Syrjä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: "Ville Syrjä" <ville.syrjala@linux.intel.com>
Cc: Hans Verkuil <hansverk@cisco.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Reviewed-by: Sean Paul <sean@poorly.run> (v1)
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
[danvet: Fix up markup: () for functions, & for structs. Style guide
also recommends to prepend struct for structures.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1559647022-7336-1-git-send-email-uma.shankar@intel.com