iris/bufmgr: Factor out GEM_MMAP ioctl from mmap_cpu and mmap_wc.

iris/bufmgr: Factor out GEM_MMAP ioctl from mmap_cpu and mmap_wc.

We want to add a new ioctl for mmap'ing buffers, so let's avoid
duplicating that code on both functions by extracting it from them
first.

[jordan.l.justen@intel.com: iris port]
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
v2: Rename helper function names (Ken).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1675>