drm/amdgpu/display: remove unused REG_OFFSET macro

The name conflicts with another macro of the same name on the ARM ixp4xx
platform, leading to build errors.
Neither of the users actually should use a name that generic, but the
other one was here first and the dc driver doesn't actually use it.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>