panfrost: Fix gl_FragColor lowering

The gl_FragColor lowering in the fragment shader depends on the number
of render targets, which can change every set_framebuffer_state.
set_framebuffer_state thus needs to force a rebind of the fragment
shader.

Fixes a regression in Piglit fbo-drawbuffers-none gl_FragColor -auto
-fbo from enabling AFBC on Mali G52.

Fixes: 28ac4d1e ("panfrost: Call nir_lower_fragcolor based on key")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!13498>
47 jobs for !13498 with fragcolor-fix in 17 minutes and 47 seconds (queued for 22 seconds)
latest merge request