drm/amdgpu: bypass tiling flag check in virtual display case (v2)

vkms leverages common amdgpu framebuffer creation, and
also as it does not support FB modifier, there is no need
to check tiling flags when initing framebuffer when virtual
display is enabled.

This can fix below calltrace:

amdgpu 0000:00:08.0: GFX9+ requires FB check based on format modifier
WARNING: CPU: 0 PID: 1023 at drivers/gpu/drm/amd/amdgpu/amdgpu_display.c:1150 amdgpu_display_framebuffer_init+0x8e7/0xb40 [amdgpu]

v2: check adev->enable_virtual_display instead as vkms can be
	enabled in bare metal as well.

Signed-off-by: Leslie Shi <Yuliang.Shi@amd.com>
Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Guchun Chen

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

@@ -1141,7 +1141,7 @@ int amdgpu_display_framebuffer_init(struct drm_device *dev,
 	if (ret)
 		return ret;
 
-	if (!dev->mode_config.allow_fb_modifiers) {
+	if (!dev->mode_config.allow_fb_modifiers && !adev->enable_virtual_display) {
 		drm_WARN_ONCE(dev, adev->family >= AMDGPU_FAMILY_AI,
 			      "GFX9+ requires FB check based on format modifier\n");
 		ret = check_tiling_flags_gfx6(rfb);