anv: flag BO for write combine when CPU visible and potentially in lmem

anv: flag BO for write combine when CPU visible and potentially in lmem

This should fix a performance regression with the internal kernel
branch which does not support the upstream I915_MMAP_OFFSET_FIXED.

With I915_MMAP_OFFSET_FIXED we defer the mapping flags to the kernel
since it knows better where buffers are going to end up (lmem or smem).

The internal kernel doesn´t have that and there we should use write
combined for anything that can be in lmem.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18841>