drm/amdgpu/vkms: drop redundant set of fb_modifiers_not_supported

Due to a coding typo.

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