staging: unisys: visorinput: remove need for 'depends on FB'

Previously, we used a hack to determine the max x,y resolution of the
visor virtual mouse: we just looked at the resolution of the
first-registered framebuffer device, using the currently-valid assumption
that in a Unisys s-Par guest environment the video will be provided by an
efifb framebuffer device.

This hack has been removed, by instead determining the default mouse
resolution by looking at fields within the visor mouse channel memory,
mouse.x_res and mouse.y_res.  If these fields are 0, a default resolution
of 1024x768 is assumed.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>