drm/radeon: implement common cs packet parse function

CS packet parse functions have a lot of in common across
all ASICs. Implement a common function and take care of
small differences between families inside the function.

This patch is a prep for major refactoring and consolidation
of CS parsing code.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>