drm: edid: Refactor HDMI VSDB detection

There are two slightly different pieces of code for HDMI VSDB
detection. Unify the code into a single helper function.

Also fix a bug where drm_detect_hdmi_monitor() would stop looking
for the HDMI VSDB after the first vendor specific block is found,
whether or not that block happened to be the HDMI VSDB. The
standard allows for any number of vendor specific blocks to be
present.

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>