drm/amdgpu: Add and use seperate reg headers for dcn302

Currently we are using dcn3 reg headers for dcn302. The offsets are
different between the two so they need seperate headers.

Add dcn302 header files and use these instead of dcn3 header

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