drm/i915: make mappable struct resource centric

Now that we are using struct resource to track the stolen region, it is
more convenient if we track the mappable region in a resource as well.

v2: prefer iomap and gmadr naming scheme
    prefer DEFINE_RES_MEM

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171211151822.20953-8-matthew.auld@intel.com