Fix wrong transition in dEQP-VK.glsl.builtin_var.fragdepth

Fix wrong transition in dEQP-VK.glsl.builtin_var.fragdepth

The test was using vkCmdClearDepthStencilImage to clear an
image, than used this image as a depth attachment in
redering performed by VulkanDrawContext utility.

However, this utility always starts the renderpass on depth
attachment from UNDEFINED layout. As no depth clear is done
in the renderpass, the depth attachment contents are undefined
(and initially cleared image contents may be corrupted).

The easiest fix here is to move the clear to LOAD_OP - as
is already done for color, leaving the UNDEFINED layout as is.

Affects: dEQP-VK.glsl.builtin_var.fragdepth*

Components: Vulkan
VK-GL-CTS issue: 559

Change-Id: Icef3289ba62f71bf1ed989f84085a4d9d3b7218a