r600g: move CB_TARGET_MASK setup into new cb_misc_state

r600g: move CB_TARGET_MASK setup into new cb_misc_state

to remove some overhead from draw_vbo. This is a derived state.

BTW, I've got no idea how compute interacts with 3D here, but it should
use cb_misc_state, so that 3D and compute don't conflict.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>