vc4: Fix clear color setup for RGB565.
vc4: Fix clear color setup for RGB565.

The util_pack_color() thing only sets up the low bits of the union, so
only return them, too.  Fixes intermittent failure on
fbo-alphatest-formats and es3conform's framebuffer-objects test under
simulation.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler