drm/i915: introduce mem->reserved

drm/i915: introduce mem->reserved

In the following patch we need to reserve regions unaccessible to the
driver during initialization, so add mem->reserved for collecting such
regions.

v2: turn into an actual intel_memory_region_reserve api

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210127131417.393872-4-matthew.auld@intel.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>