panfrost: Extend clear colour packing

panfrost: Extend clear colour packing

Eventually, this will allow packing clear colours for all formats,
including floating-point framebuffers, pure integer buffers, and special
formats. Currently, a few of these formats are supported, and many more
are handled through a generic Gallium colour packing path (which is not
a perfect fit for the hardware, but works for many formats and is a sane
default for the moment.)

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