drm/i915: remove unused config DRM_I915_UNSTABLE

Essentially this is a revert of commit d9d54a53 ("drm/i915: Put
future HW and their uAPIs under STAGING & BROKEN").

We currently have no users for this config option. The last one was
removed in commit 8c26491f ("drm/i915: Kill the fake lmem
support"). Drop it altogether; it's easy enough to resurrect if need
arises.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230327105330.312131-2-jani.nikula@intel.com