drm/edid: Clear old dvi_dual/max_tmds_clock before parsing the new EDID

Clear out old max_tmds_clock and dvi_dual information (possibly from a
previous EDID) before parsing the current EDID. Tne current EDID might
not even have these in its HDMI VSDB, which would mean that we'd leave
the old stale values in place.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1475070703-6435-3-git-send-email-ville.syrjala@linux.intel.com