parisc: fbdev/stifb: Align graphics memory size to 4MB

commit aca7c13d upstream.

Independend of the current graphics resolution, adjust the reported
graphics card memory size to the next 4MB boundary.
This fixes the fbtest program which expects a naturally aligned size.

Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>