vknulldrv: Fix several entry point prototypes

Entry point implementations were never used where the appropriate
PFN_vk* type was expected, so when the prototypes were wrong the
compiler wouldn't know or be able to tell us. Of course, this means
several of them were wrong after the update to header 0.170.2.

This change adds an explicit static_cast to the appropriate function
pointer type for each function so the compiler will issue an error
when the prototype is wrong. It also fixes all of the resulting
errors.

Change-Id: I44211353476b56c8b429ee5d7ca7a669e8a95afd
(cherry picked from commit 90018472e9c4eed2fae226bd64f29da065d4b1e7)