drm/i915: Split i915_gem_init_stolen()

Add some helpers: adjust_stolen(), request_smem_stolen_() and
init_reserved_stolen() that are now called by i915_gem_init_stolen() to
initialize each part of the Data Stolen Memory region.

Main goal is to split the reserved part within the stolen, also known as
WOPCM, as its calculation changes often per platform and is a big source
of confusion when handling stolen memory.

Reviewed-by: Wayne Boyer <wayne.boyer@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220915-stolen-v2-2-20ff797de047@intel.com