vulkan/wsi/x11: Fix behavior of vkGetPhysicalDeviceSurfacePresentModesKHR

x11_surface_get_present_modes() is currently asserting that the number of
elements in pPresentModeCount must be greater than or equal to the number
of present modes available. This is buggy because pPresentModeCount
elements are later copied from the internal modes' array, so if
pPresentModeCount is greater, it will overflow it.

On top of that, this assertion violates the spec. From the Vulkan 1.0
(revision 32, with KHR extensions), page 581 of the PDF:

    "If the value of pPresentModeCount is less than the number of
     presentation modes supported, at most pPresentModeCount values will be
     written. If pPresentModeCount is smaller than the number of
     presentation modes supported for the given surface, VK_INCOMPLETE
     will be returned instead of VK_SUCCESS to indicate that not all the
     available values were returned."

So, the correct behavior is: if pPresentModeCount is greater than the
internal number of formats, it is clamped to that many present modes. But
if it is lesser than that, then pPresentModeCount elements are copied,
and the call returns VK_INCOMPLETE.

This fix is similar (but simpler and more readable) than the one I provided
in 750d8cad72a for vkGetPhysicalDeviceSurfaceFormatsKHR, which was suffering
from the same problem.

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>