etnaviv: add missing fallthrough annotation
etnaviv: add missing fallthrough annotation

Caught by Coverity, reported to me by imirkin.

Signed-off-by: Christian Gmeiner <7d98dcb0e8545320ffc03d45630c91ed21ec4c41@gmail.com>
Reviewed-by: Eric Engestrom <de0b63cf2dfb1fae8ed3c3b4244856a05677d4f0@imgtec.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler