radeonsi/compute: Allocate the scratch buffer during state creation

radeonsi/compute: Allocate the scratch buffer during state creation

This moves scratch buffer allocation from si_launch_grid() to
si_create_compute_state().  This helps to reduce the overhead of
launching a kernel and also fixes a bug in the code that would cause
the scratch buffer to be too small if a kernel with smaller scratch size
was launched before a kernel with a larger scratch size.

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