radv: set fmask state to all 0s when no fmask. (v2)

radv: set fmask state to all 0s when no fmask. (v2)

The shader reads the descriptor to decide if it should take the
fmask value, however we weren't initing it always, which meant
random crap, esp with MSAA depth textures.

Fixes random hangs with:
dEQP-VK.glsl.builtin_var.fragdepth.*

v2: check fmask_state is not NULL

Fixes: f4e499ec791 "radv: add initial non-conformant radv vulkan driver"
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>