drm/ast: Initialized data needed to map fbdev memory

drm/ast: Initialized data needed to map fbdev memory

commit 28fb4cb7fa6f63dc2fbdb5f2564dcbead8e3eee0 upstream.

Due to a missing initialization there was no way to map fbdev memory.
Thus for example using the Xserver with the fbdev driver failed.
This fix adds initialization for fix.smem_start and fix.smem_len
in the fb_info structure, which fixes this problem.

Requested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Egbert Eich <eich@suse.de>
[pulled from SuSE tree by me - airlied]
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>