drm/mediatek: Fix warning about unhandled enum value

Fixes the following build warning:
drivers/gpu/drm/mediatek/mtk_hdmi.c:327:2: warning: enumeration value ‘HDMI_INFOFRAME_TYPE_DRM’ not handled in switch [-Wswitch]

Introduced with the addition of HDMI_INFOFRAME_TYPE_DRM in the commit
below, but the code really should have been future-proofed from the
start.

Fixes: 2cdbfd66 ("drm: Enable HDR infoframe support")
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Ville Syrjälä <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 Syrjälä" <ville.syrjala@linux.intel.com>
Cc: Hans Verkuil <hansverk@cisco.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Acked-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522202207.223110-1-sean@poorly.run