radeonsi: add flexible shader descriptor management and use it for sampler views

radeonsi: add flexible shader descriptor management and use it for sampler views

It moves all sampler view descriptors to a buffer.
It supports partial resource updates and it can also unbind resources
(required for FMASK texturing).

The buffer contains all sampler view descriptors for one shader stage,
represented as an array. On top of that, there are N arrays in the buffer,
which are used to emulate context registers as implemented by the previous
ASICs (each array is a context).

This uses the RCU synchronization approach to avoid read-after-write hazards
as discussed in the thread:
"radeonsi: add FMASK texture binding slots and resource setup"

CP DMA is used to clear the descriptors at context initialization and to copy
the descriptors from one context to the next.

v2: - use PKT3_DMA_DATA on CIK (I'll test CIK later)
    - turn the bool CP DMA parameters into self-explanatory flags
    - add a nice simple API for packet emission to radeon_winsys.h
    - use 256 contexts, 128 causes texture corruption in openarena