drm: add documentation for tv connector state margins

A bit boring documentation fix, but gets rid of the warnings:

./include/drm/drm_connector.h:370: warning: Function parameter or member 'margins.left' not described in 'drm_tv_connector_state'
./include/drm/drm_connector.h:370: warning: Function parameter or member 'margins.right' not described in 'drm_tv_connector_state'
./include/drm/drm_connector.h:370: warning: Function parameter or member 'margins.top' not described in 'drm_tv_connector_state'
./include/drm/drm_connector.h:370: warning: Function parameter or member 'margins.bottom' not described in 'drm_tv_connector_state'

[Depends on fe7bc493 ("scripts: kernel-doc: support in-line comments
on nested structs/unions") in docs-next to actually fix the warnings.]

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180219104009.4887-1-jani.nikula@intel.com