freedreno/a4xx: point regid to "red" even for alpha-only rb formats
freedreno/a4xx: point regid to "red" even for alpha-only rb formats

Looks like a4xx hw does this in a more standard way and we don't need to
hack around it like we do on a3xx. Fixes GL_ALPHA formats in
fbo-blending-formats, fbo-colormask-formats, and fbo-alphatest-formats.

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Cc: 59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler