drm/amdgpu: rename macro for VCN2.0 2.5 and 3.0

Rename SOC15_DPG_MODE_OFFSET_2_0, RREG32_SOC15_DPG_MODE_2_0 and
WREG32_SOC15_DPG_MODE_2_0 for VCN2.0, VCN2.5 and VCN3.0.
These three macros are used VCN2.0, VCN2.5 and VCN3.0, therefore rename
it to be a general name.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Reviewed-by: James Zhu <james.zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>