radeonsi: Use FP16 shader export format when necessary / possible.

radeonsi: Use FP16 shader export format when necessary / possible.

Fixes piglit fbo-blending-formats.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>