drm/i915: lookup for mem_region of a mem_type

Lookup function to retrieve the pointer to a memory region of
a mem_type.

v2:
  for_each_memory_region is used.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200104191043.2207314-1-chris@chris-wilson.co.uk