staging: fbtft: fb_ssd1306: Support smaller screen sizes

There is 64x48 display exists. In order to support that set multiplexer
to 48 pixels and window address to proper position in the graphic display
data RAM.

Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>