Vulkan: only report refreshDuration, not {min|max}RefreshDuration

Test: Manually tested with a modified cube demo, that reports the refresh
duration returned from this extension.

As part of discussions within Khronos, it was decided that
VK_GOOGLE_display_timing should only report one refresh duration for a display,
and assume that it's refresh rate is fixed (which is the case for our current
devices).

Change-Id: I772348281c18a36b02dcfe0519d1943e25a41f7c