Sets up the GFX ring and loads ucode for GFX and Compute.

Todo:
- handle compute queue setup.

v2: add documentation
v3: integrate with latest reset changes
v4: additional init fixes
v5: scratch reg write back no longer supported on CIK
v6: properly set CP_RB0_BASE_HI
v7: rebase
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>