drm/amdgpu: Enable scatter gather display support

Enables sg display if vram size <= THRESHOLD(256M); otherwise
still use vram as display buffer.
This patch fixed some potention issues introduced by change
"allow framebuffer in GART memory as well" due to CZ/ST hardware
limitation.

v2: Change default setting to auto.
v3: Move some logic from amdgpu_display_framebuffer_domains()
    to pin function, suggested by Christian.
v4: Split into several patches.
v5: Drop module parameter for now.
Signed-off-by: Samuel Li <Samuel.Li@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -138,6 +138,7 @@ extern int amdgpu_si_support;
 extern int amdgpu_cik_support;
 #endif
 
+#define AMDGPU_SG_THRESHOLD			(256*1024*1024)
 #define AMDGPU_DEFAULT_GTT_SIZE_MB		3072ULL /* 3GB by default */
 #define AMDGPU_WAIT_IDLE_TIMEOUT_IN_MS	        3000
 #define AMDGPU_MAX_USEC_TIMEOUT			100000	/* 100 ms */

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

@@ -701,6 +701,15 @@ int amdgpu_bo_pin_restricted(struct amdgpu_bo *bo, u32 domain,
 			return -EINVAL;
 	}
 
+	/* This assumes only APU display buffers are pinned with (VRAM|GTT).
+	 * See function amdgpu_display_supported_domains()
+	 */
+	if (domain == (AMDGPU_GEM_DOMAIN_VRAM | AMDGPU_GEM_DOMAIN_GTT)) {
+		domain = AMDGPU_GEM_DOMAIN_VRAM;
+		if (adev->gmc.real_vram_size <= AMDGPU_SG_THRESHOLD)
+			domain = AMDGPU_GEM_DOMAIN_GTT;
+	}
+
 	if (bo->pin_count) {
 		uint32_t mem_type = bo->tbo.mem.mem_type;