drm/i915: add struct i915_dsm to wrap dsm members together

Wrap the stolen memory related struct drm_i915_private members (dsm,
dsm_reserved, and stolen_usable_size) together in a a new struct
i915_dsm.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230116173422.1858527-1-jani.nikula@intel.com