To avoid some circular dependencies due to pvr_private.h, split out
pvr_framebuffer etc into their own header.
We often only need to peek into the framebuffer, so this seems like a
good idea anyway. We can reconsider this once dynamic rendering has
landed, and we know how much remains here.
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37432>