mesa: don't always set _NEW_PROGRAM when linking

mesa: don't always set _NEW_PROGRAM when linking

We only need to set it when linking was successful and the program
being linked is currently active.

The programs_in_use mask is just used as a flag for now but in
a future change we will use it to update the CurrentProgram array.

V2: make sure to flush vertices before linking (suggested by Marek)

Reviewed-by: Marek Olšák <marek.olsak@amd.com>