drm: read EDID extensions from monitor

Usually drm read basic EDID, that is enough for us, but since igital display
were introduced i.e. HDMI monitor, sometime we need to interact with monitor by
EDID extension information,

EDID extensions include audio/video data block, speaker allocation and vendor specific data blocks.

This patch intends to read EDID extensions from digital monitor for users.

Signed-off-by: Ma Ling <ling.ma@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>