drm/doc/rfc: drop lmem uapi section

We still have quite a bit more work to do with overall reworking of
the ttm-based dg1 code, but the uapi stuff is now finalized with the
latest pull. So remove that.

This also fixes kerneldoc build warnings because we've included the
same headers in two places, resulting in sphinx complaining about
duplicated symbols. This regression has been created when we moved the
uapi definitions to the real include/uapi/ folder in 727ecd99
("drm/doc/rfc: drop the i915_gem_lmem.h header")

v2: Fix a few references that I missed, the htmldocs build took
forever.

Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Tested-by Stephen Rothwell <sfr@canb.auug.org.au> (v1)
References: https://lore.kernel.org/dri-devel/20210603193242.1ce99344@canb.auug.org.au/


Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 727ecd99 ("drm/doc/rfc: drop the i915_gem_lmem.h header")
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210810142748.1983271-1-daniel.vetter@ffwll.ch