The vulkan spec says the following about vkMapMemory:

"ppData is a pointer to a void * variable in which is returned a
 host-accessible pointer to the beginning of the mapped range. This
 pointer minus offset must be aligned to at least
 VkPhysicalDeviceLimits::minMemoryMapAlignment."

So let's report the same value as the gallium-driver reports, otherwise
we'll fail to adhere to the alignment requirement.

This fixes a few Piglit failures for Zink.

Closes: mesa/mesa#4267


Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <mesa/mesa!8965>