drm/amd: Add ucode DMCU support

DMCU (Display Microcontroller Unit) is a GPU chip involved in
eDP features like Adaptive Backlight Modulation and Panel Self
Refresh.

DMCU has two pieces of firmware: the ERAM and the interrupt
vectors, which must be loaded seperately.

To this end, the DMCU firmware has a custom header and parsing
logic similar to MEC, to extract the two ucodes from a single
struct firmware.

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>