drm/amdgpu: simplify Raven, Raven2, and Picasso handling

Treat them all as Raven rather than adding a new picasso
asic type.  This simplifies a lot of code and also handles the
case of rv2 chips with the 0x15d8 pci id.  It also fixes dmcu
fw handling for picasso.

Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>