drm/amd/display: Add DCN2 clk mgr

Adds support for handling of clocking relevant to the DCN2 block,
including programming of the DCCG (Display Controller Clock Generator)
block:

HW Blocks:

    +--------+       +--------+
    |  DIO   |       |  DCCG  |
    +--------+       +--------+

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>