hmi-controller: fix wrong panel width

The width of the first base layer is used for all panels.
Every display has a base layer which is as big as the
display. Therefore, it is wrong to use the width of the
first base layer for all panels, because every display
could have a different resolution.

Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>