Draft: panvk: Add support for indirect draws and fix indexed draws

This MR contains a bunch of fixes to pan_indirect_draw.c and is using this infrastructure to support indirect draws in panvk.

Note that indexed draws are now implemented as indirect draws to let the GPU retrieve the vertex range and get rid of the WARNING: Crawling index buffers from the CPU isn't valid in Vulkan warning.

Marked WIP because I still need to make sure my changes don't regress stuff in the GL driver...

/cc @jekstrand since you worked on panvk's indexed draw logic recently ;-)