hw/display/artist: Refactor x/y coordination extraction

hw/display/artist: Refactor x/y coordination extraction

Simplify the code by using new introduced ADDR_TO_Y() and ADDR_TO_X()
macros. Those macros extract the x/y-coordinate from the given uint32.

As further simplification the extraction of the x/y coordinates for
VRAM_WRITE_INCR_Y and VRAM_WRITE_INCR_X can be done centrally in
vram_bit_write(), so move this code up into the function.

ADDR_TO_Y() is still limited to 10 bits which allow to address up to of
1024 lines - this will be increased in a follow-up patch.

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