Instead of passing lots of RendererVk member variables into
GenerateCaps(), do the work in a member function.

BUG=angleproject:2672

Change-Id: Icf16f3388174ddb676272ec0fa76a288ce2d1e4e
Reviewed-on: https://chromium-review.googlesource.com/c/1463959Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>