drm/amd/display: Only define DP 2.0 symbols if not already defined

drm/amd/display: Only define DP 2.0 symbols if not already defined

[Why]
For some reason we're defining DP 2.0 definitions inside our
driver. Now that patches to introduce relevant definitions
are slated to be merged into drm-next this is causing conflicts.

In file included from drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c:33:
In file included from ./drivers/gpu/drm/amd/amdgpu/../amdgpu/amdgpu.h:70:
In file included from ./drivers/gpu/drm/amd/amdgpu/../amdgpu/amdgpu_mode.h:36:
./include/drm/drm_dp_helper.h:1322:9: error: 'DP_MAIN_LINK_CHANNEL_CODING_PHY_REPEATER' macro redefined [-Werror,-Wmacro-redefined]
        ^
./drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dp_types.h:881:9: note: previous definition is here
        ^
1 error generated.

[How]
Guard all display driver defines with #ifndef for now. Once we pull
in the new definitions into amd-staging-drm-next we will follow
up and drop definitions from our driver and provide follow-up
header updates for any addition DP 2.0 definitions required
by our driver.

We also ensure drm_dp_helper.h is included before dc_dp_types.h.

v3: Ensure drm_dp_helper.h is included before dc_dp_types.h

v2: Add one missing endif

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>