panvk: Fix FrontFacing, PointCoord and FragCoord handling
Bifrost and Midgard don't expose FrontFacing, PointCoord and FragCoord the same way, on Bifrost all of those are sysvals, and on Midgard they are treated as varyings.
v1 of this MR was extending spirv_to_nir_options to let the backend select how FrontFacing and PointCoord should be retrieved, but @jekstrand suggested to let drivers do the sysval -> varying conversion on their own.