vk: Always use a placeholder vertex shader in meta

vk: Always use a placeholder vertex shader in meta

The clear pipeline didn't have a vertex shader and relied on the clear
shader being hardcoded by the compiler to accept one attribute. This
necessitated a few special cases in the 3DSTATE_VS setup. Instead,
always provide a vertex shader, even if we disable VS dispatch.

Signed-off-by: Kristian Høgsberg Kristensen <kristian.h.kristensen@intel.com>