drm/amdgpu/gmc: set a default disable value for AGP

To disable AGP, the start needs to be set to a higher
value than the end.  Set a default disable value for
the AGP aperture and allow the IP specific GMC code
to enable it selectively be calling amdgpu_gmc_agp_location().

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
index ab234bf..a0023d1 100644
--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c
@@ -1256,7 +1256,7 @@ static void mmhub_read_system_context(struct amdgpu_device *adev, struct dc_phy_
 	agp_top = adev->gmc.agp_end >> 24;
 
 	/* AGP aperture is disabled */
-	if (agp_bot == agp_top) {
+	if (agp_bot > agp_top) {
 		logical_addr_low = adev->gmc.fb_start >> 18;
 		if (adev->apu_flags & AMD_APU_IS_RAVEN2)
 			/*