drm/amdgpu/gfx: Introduce helpers handling CSB manipulation

drm/amdgpu/gfx: Introduce helpers handling CSB manipulation

From GFX6 to GFX11, there is a function for getting the CSB buffer to be
put into the hardware. Three common parts are duplicated in all of these
GFX functions:

1. Prepare the CSB preamble.
2. Parser the CS data.
3. End the CSB preamble.

This commit creates helpers to be used from GFX6 to GFX11.

Signed-off-by: Rodrigo Siqueira <siqueira@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>