openfimg: compiler: Fix potential use of unitialized variable

Caught by:

external/mesa/src/gallium/drivers/openfimg/compiler/openfimg_ir.c: In
function 'merge_swizzle':
external/mesa/src/gallium/drivers/openfimg/compiler/openfimg_ir.c:463:23:
error: 'chan' may be used uninitialized in this function
[-Werror=maybe-uninitialized]
cc1: all warnings being treated as errors

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
openfimg: Allow void pointer arithmetics in Android build

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>