panfrost: Decode render target swizzle/channels

panfrost: Decode render target swizzle/channels

On MRT-capable systems, the framebuffer format is encoded as a 64-bit
word in the render target descriptor. Previously, the two 32-bit
words were exposed as opaque hex values. This commit identifies a 12-bit
Mali swizzle and a 2-bit channel counter, removing some of the magic. It
also adds decoding support for the AFBC and MSAA enable bits, which were
already known but otherwise ignored in pandecode.

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