drm/i915: move fb_tracking under display sub-struct

Move display frontbuffer tracking related members under drm_i915_private
display sub-struct.

Rename struct i915_frontbuffer_tracking to intel_frontbuffer_tracking
while at it.

FIXME: fb_tracking.lock mutex init should be moved away from
i915_gem_init_early().

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a5444d0a373afca46da9a2f6e4db442af21b429b.1661779055.git.jani.nikula@intel.com