drm/i915: make reserved struct resource centric

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

v2: s/<= end + 1/< end/ (Chris)
v3: 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/20171211151822.20953-7-matthew.auld@intel.com