i965/drm: Make register write check handle execbuffer directly.

i965/drm: Make register write check handle execbuffer directly.

I'm about to rewrite how relocation handling works, at which point
drm_bacon_bo_emit_reloc() and drm_bacon_bo_mrb_exec() won't exist
anymore.  This code is already largely not using the batchbuffer
infrastructure, so just go all the way and handle relocations, the
validation list, and execbuffer ourselves.  That way, we don't have
to think the weird case where we only have a screen, and no context,
when redesigning the relocation handling.

v2: Write reloc.presumed_offset + reloc.delta into the batch, rather
    than duplicating the comment, so it's obvious that they match
    (suggested by Chris).  Also add a comment about why we don't do
    any error checking.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>