cirrus: avoid write only variables

Compiling with GCC 4.6.0 20100925 produced a lot of warnings like:
In file included from /src/qemu/hw/cirrus_vga_rop.h:174:0,
                 from /src/qemu/hw/cirrus_vga.c:284:
/src/qemu/hw/cirrus_vga_rop2.h: In function 'cirrus_patternfill_0_8':
/src/qemu/hw/cirrus_vga_rop2.h:48:18: error: variable 'col' set but not used [-Werror=unused-but-set-variable]
/src/qemu/hw/cirrus_vga_rop2.h: In function 'cirrus_colorexpand_transp_0_8':
/src/qemu/hw/cirrus_vga_rop2.h:104:18: error: variable 'col' set but not used [-Werror=unused-but-set-variable]

Fix the warnings by introducing an inline function, which avoids
exposing write-only variables.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
