glsl: Set UsesDFdy appropriately for GLSL shaders.

glsl: Set UsesDFdy appropriately for GLSL shaders.

This patch updates the ir_set_program_inouts_visitor so that it also
sets gl_fragment_program::UsesDFdy.

This is a bit of a hack (since dFdy() isn't an input or an output),
but there's no other obvious visitor to squeeze this functionality
into, and it would be silly to create a brand new visitor just for
this purpose.

v2: use local 'fprog' var to avoid repeated casting.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>