drm/amdgpu: add asic callback to get memsize register

Newer asics use different registers so abstract it.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -713,7 +713,7 @@ bool amdgpu_need_post(struct amdgpu_device *adev)
 		return true;
 	}
 	/* then check MEM_SIZE, in case the crtcs are off */
-	reg = RREG32(mmCONFIG_MEMSIZE);
+	reg = amdgpu_asic_get_config_memsize(adev);
 
 	if (reg)
 		return false;