drm/i915: Move stolen memory handling into i915_gem_stolen

We are preparing for Xe. Xe stolen memory handling differs from i915 so we
want to move stolen memory handling details into i915_gem_stolen.

Also add a common type for fbc compressed fb and use it from fbc code
instead of underlying type directly. This way we can have common type
i915_stolen_fb for both i915 and Xe.

v2: Fix couple of checkpatch warnings

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230614051731.745821-2-jouni.hogander@intel.com