loader: Fix a 1.0/1.1 versioning bug

Fix a bug where when mixing a 1.0 and a 1.1 driver, if the loader
stores the 1.0 driver first, the apiVersion field in vkCreateInstance
will be rewritten to 1.0 for both drivers. Now it will only be
changed for the 1.0 driver, as intended.

Change-Id: I9e7c5e0b0b7c0eb660082319dcddd9e8ef8adc61