drm: Drop a redundant unused variable

Drop a redundant and unused variable "hdr_output_metadata" from
drm_connector.

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1559159944-21103-2-git-send-email-uma.shankar@intel.com