radeonsi: use TC L2 for CP DMA operations with shader resources on CIK

radeonsi: use TC L2 for CP DMA operations with shader resources on CIK

So that TC L2 doesn't need to be flushed.

The only problem is with index buffers, which don't use TC.
A simple solution is added that flushes TC L2 before a draw call (TC_L2_dirty).

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