dxva2: Pass variable of correct type to IDirectXVideoDecoder_GetBuffer()

This avoids related incompatible pointer type warnings.
dxva2: Clean up definition of _WIN32_WINNT

Only set a value if _WIN32_WINNT is undefined or smaller than 0x0600. This is
cleaner than unconditional definition and avoids a number of redefinition
warnings. Also only define a value in one of the two dxva2 headers.