drm/amd: Init modifier field of helper fb.

Otherwise the field ends up being used uninitialized when
enabling modifiers, failing validation with high likelyhood.

v4: Use memset
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
(for v1)
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -207,6 +207,7 @@ static int amdgpufb_create(struct drm_fb_helper *helper,
 	int ret;
 	unsigned long tmp;
 
+	memset(&mode_cmd, 0, sizeof(mode_cmd));
 	mode_cmd.width = sizes->surface_width;
 	mode_cmd.height = sizes->surface_height;