radeonsi: set number of userdata SGPRs of GS copy shader to 4

radeonsi: set number of userdata SGPRs of GS copy shader to 4

It only needs the constant buffer with clip planes and read-write resources
for the GS->VS ring and streamout. That's 2 pointers.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>