gallium: give fragment-shader derivatives its own cap

gallium: give fragment-shader derivatives its own cap

Shader Model 3.0 is a big promise to make to the state-tracker, and
for instance mobile hardware might support fragment-shader derivatives
but not some of the other features of SM3. So let's give this its own
cap for simplicity.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>