freedreno/a6xx: small texture emit cleanup

freedreno/a6xx: small texture emit cleanup

Prep work for fb_read (blend_equation_advanced)

Switch to using 'enum pipe_shader_type' everywhere, and (optional, in
non-cache / slowpath case) pass ctx instead of image/ssbo state.  In the
fb_read case we also need to access the framebuffer state, so having
the ctx simplifies things.

Signed-off-by: Rob Clark <robdclark@chromium.org>