vulkan/wsi: fix assert failure of failed to find a supported modifier

For the fail case, in function wsi_wl_surface_create_swapchain, num_modifier_lists is set to 1, while num_drm_modifiers is 0. And it leads to image_modifier_count in function wsi_configure_native_image is 0. So the 'assert(!"Failed to find a supported modifier!' is hit.

Move the modifier count check to the 1st place to avoid the failure.

Signed-off-by: Shuicheng Lin shuicheng.lin@intel.com