v3d: Fix SRC_ALPHA_SATURATE blending for RTs without alpha.

Fixes
dEQP-GLES3.functional.fragment_ops.blend.default_framebuffer.rgb_func_alpha_func.dst.src_alpha_saturate_src_alpha_saturate
and friends with --deqp-egl-config-name=rgb565d0s0

Cc: "18.2" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit f73f7483)