gallium: Add a new A4B4G4R4 pipe format for Broadcom.

gallium: Add a new A4B4G4R4 pipe format for Broadcom.

The VC5 HW puts A in the low bits and R in the high bits.  We can't just
swizzle in the shaders because the blending HW can't pick what channel A
is in, so make a new format to match it.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>