pan/decode: Check for MFBD preload chicken bit

pan/decode: Check for MFBD preload chicken bit

If this bit is clear, MFBD preload will be enabled, and you.. don't want
that. (At least, when the bit is clear, the old contents of the
framebuffer will be preserved. I'm assuming this is what "MFBD preload"
refers to in kbase.)

Validate that this bit is always set.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>