drm/msm: fix Y420CMDB EDID block parsing

Fix the Y420CMDB EDID block parsing to handle
the case where the block does not include a capability
bitmap.

This case means that all the short video descriptors
support the YUV 420 mode as well.

Also fix the incorrect length check in the parser API.
The capability bitmap has no minimum length field.

Change-Id: I5d9c2d3ac11d5ddad8e36cb7acfebfb41175f4b7
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>