drm/edid: Fix parsing of EDID 1.4 Established Timings III descriptor

The EDID 1.4 specification section 3.10.3.9 defines an Established Timings III
descriptor (tag #F7h). The parsing of this descriptor by drm_est3_modes() is
off by one byte: the offset of the first timing bitmap is 6, not 5.

Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160328002258.E75DF6E35D@gabe.freedesktop.org