drm: edid: Add some bounds checking

Make sure drm_detect_hdmi_monitor() and drm_detect_monitor_audio() don't
access beyond the extension block.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>