anv/formats: Refactor anv_get_format
anv/formats: Refactor anv_get_format

The new code removes the switch statement and instead handles depth/stencil
as up-front special cases.  This allows for potentially more complicated
color format handling in the future.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler