radeonsi: rework how shader pointers to descriptors are set

radeonsi: rework how shader pointers to descriptors are set

This is mainly needed for tessellation where a VS can be bound as VS, ES,
or LS, and TES (tess. evaluationshader) can be bound as VS or ES or neither.
Therefore we need the ability to move pointers to descriptors between
shaders arbitrarily.

The idea is that the context has a mapping from PIPE_SHADER_x to
SPI_SHADER_USER_DATA_x. After a shader is enabled or disabled,
si_shader_change_notify should be called to update this mapping accordingly.

There is a dirty flag for each shader pointer, but only one emit function
for all pointers in the whole context, whose code and logic is separated
from descriptors.

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