When handling dirty bindings, we were treating binding indices as if
they were attribute indices, causing strange behavior. This change fixes
the dirty bit handler to use the provided index properly and update the
attributes that correspond to the binding index.

Test: ./angle_end2end_tests --gtest_filter='VertexAttributeTestES31.OnlyUpdate*/ES3_1_Vulkan'
Bug: angleproject:3598
Change-Id: I2bc3cd4da5f639a1301776fcf057a74368812c56
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1718786
Commit-Queue: James Dong <dongja@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>