drm/gem: Rename drm_gem_dumb_map_offset() to drm_gem_map_offset()

drm_gem_dumb_map_offset() is a useful helper for non-dumb clients, so
rename it to remove the _dumb and add a comment that it can be used by
shmem clients.

Signed-off-by: Steven Price <steven.price@arm.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190627155318.38053-2-steven.price@arm.com