drm/i915/fbc: Extract intel_fbc_stolen_end()

Declutter find_compression_limit() a bit by extracting
intel_fbc_stolen_end().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210610183237.3920-8-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>