drm/gem: rip out drm vma accounting for gem mmaps

Doesn't really add anything which can't be figured out through
proc files. And more clearly separates the new gem mmap handling
code from the old drm maps mmap handling code, which is surely a
good thing.

Cc:  Martin Peres <martin.peres@free.fr>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>