drm/cirrus: use drm_set_preferred_mode

Explicitly set 1024x768 as default mode, so the display doesn't come up
with the largest supported mode.

While being at it drop first three drm_add_modes_noedid calls.  As
drm_add_modes_noedid fills the mode list with modes from the database
*up to* the specified size it is pretty pointless to call it multiple
times with different sizes.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>