hw/display/artist: Verify artist screen resolution

hw/display/artist: Verify artist screen resolution

Artist hardware is limited to 2048 x 2048 pixels.
STI ROMs allow at minimum 640 x 480 pixels.

Qemu users can adjust the screen size on the command line with:
 -global artist.width=800 -global artist.height=600
but we need to ensure that the screen size stays inside the given
boundaries, otherwise print an error message and adjust.

Signed-off-by: Helge Deller <deller@gmx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>