vkr: work around a Use-of-uninitialized-value
Generated by https://gitlab.freedesktop.org/olv/venus-protocol/-/merge_requests/22.
VkDrmFormatModifierPropertiesListEXT::drmFormatModifierCount may be used
uninitialized. It is a codegen bug but a proper fix breaks the
protocol. Until we are ready to finalize the protocol, let's work
around it.
It works so far because Mesa calls the function twice in a row. In the
first call, pDrmFormatModifierProperties is NULL and the uninitialized
value is not used. Instead, it is initialized by the host driver.
In the second call, because of how the temp pool works, the memory gets reused and the "uninitialized value" is already initialized. Thanks goes to Yiwei for figuring this out.