drm/vkms: Drop custom vkms_dumb_map().

This is the same as the default drm_gem_dumb_map_offset()
implementation, except that this one missed the ban on userspace
mapping an imported dmabuf (which seems like intended common behavior
for drivers).

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181126215929.20546-1-eric@anholt.net
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>