drm/edid: Return DispID length from drm_find_displayid_extension()

drm/edid: Return DispID length from drm_find_displayid_extension()

As with the byte offset (idx) drm_find_displayid_extension() is
the only one who actually knows how much data the resulting DispID
block can contain. So return the length from therein instead of
assuming it's the EDID block length all over.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200313162054.16009-5-ville.syrjala@linux.intel.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>