drivers/video/bus_vcxk.c: Fix GCC 4.6 warning

Fix:
bus_vcxk.c: In function 'vcxk_display_bitmap':
bus_vcxk.c:396:16: warning: variable 'compression' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>