panfrost: Prepare Gallium driver for per-gen compilation and per-gen XML

No functional changes, just shuffling code around to make it possible. After this change set, only pan_cmdstream.c has access to GenXML, and the routines in pan_cmdstream.c is only accessible via vtables. Modeled after iris.

Similar prep work will be needed on panvk, and more frustratingly on the common code. I'm still trying to figure out the right way to handle common stuff. One approach is to keep the common pack code in static inline functions in headers, which then get #included from pan_cmdstream.c (/ panvk equivalent). Another option is to recompile the common code per-gen like we will do for pan_cmdstream. Yet another option is to vent our collective frustration with the common code and copy the trivial stuff back into our respective drivers, and do one of the above options for the nontrivial stuff (mostly just the blitter).

Cc @bbrezillon