drm/amd/display: Add DCE BIOS_SCRATCH_0 register

drm/amd/display: Add DCE BIOS_SCRATCH_0 register

The BIOS uses this register to write the results of the
DAC_LoadDetection command, so we'll need to read this
in order to make DAC load detection work.

As a reference, I used the mmBIOS_SCRATCH_0 definition from
the amdgpu legacy display code.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>