drm/amdgpu/soc15: fix nullptr issue in soc15_read_register() for reg base accessing

The failed case is no SDMA1 IP for Renoir discovery table while in accessing SDMA1 reg base,
thus need have nullptr test for soc15_read_register invoked in MMR addres space inquire opt.
Signed-off-by: Prike.Liang <Prike.Liang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/soc15.c

@@ -415,7 +415,8 @@ static int soc15_read_register(struct amdgpu_device *adev, u32 se_num,
 	*value = 0;
 	for (i = 0; i < ARRAY_SIZE(soc15_allowed_read_registers); i++) {
 		en = &soc15_allowed_read_registers[i];
-		if (reg_offset != (adev->reg_offset[en->hwip][en->inst][en->seg]
+		if (adev->reg_offset[en->hwip][en->inst] &&
+			reg_offset != (adev->reg_offset[en->hwip][en->inst][en->seg]
 					+ en->reg_offset))
 			continue;