drm/i915: Restrict DRM_I915_DEBUG to developer builds

Let's not encourage everybody to build i915's debug code, and certainly
not the build robots who need to scrutinise the production build. Since
CI will complain if the debug build is broken, having the other build
bots focus on the builds we don't cover ourselves should improve the
build coverage.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 4f86975f ("drm/i915: Add DEBUG_GEM to the recommended CI config")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210122091058.5145-1-chris@chris-wilson.co.uk


(cherry picked from commit c442f658299d59b327a4bf21457ec8ece936f133)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>