anv: don't use Element.get in anv_entrypoints_gen.py

anv: don't use Element.get in anv_entrypoints_gen.py

This has the potential to mask errors, since Element.get works like
dict.get, returning None if the element isn't found. I think the reason
that Element.get was used is that vulkan has one extension that isn't
really an extension, and thus is missing the 'protect' field.

This patch changes the behavior slightly by replacing get with explicit
lookup in the Element.attrib dictionary, and using xpath to only iterate
over extensions with a "protect" attribute.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>