pvr: Add pbe accum fmt sizes and pack functions.

This is some ground work for vkCmdClearAttachments().

For the clear attachments implementation we'll be manually pre packing (like the USC would have done) the color values and pass them through the USC to the PBE which will do the final packing to the target image format and write it out.

This MR adds the color packing function and a function to get the size required for the accum format.