drm/doc/rfc: drop the i915_gem_lmem.h header

The proper headers have now landed in include/uapi/drm/i915_drm.h, so we
can drop i915_gem_lmem.h and instead just reference the real headers for
pulling in the kernel doc.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210511170356.430284-1-matthew.auld@intel.com