Fix AHB external memory test mip levels

Fix AHB external memory test mip levels

AHB external memory tests were testing mipmaps using a
queried maximum number of supported mipmap levels, but
at the same time using an image size of 64x64.
The number of mip levels need to match the number for a
full mip chain, in this case 7. Now a correct number of
mip levels is used, which also gets rid of a validation
error.

VK-GL-CTS Issue: 3112

Affects:

dEQP-VK.api.external.memory.android_hardware_buffer.image_formats.*

Components: Vulkan
Change-Id: I1c6d09f93c06ac1c52cea283fd86359ccde01e4f