anv: Use python style in anv_entrypoints_gen.py

anv: Use python style in anv_entrypoints_gen.py

These are all fairly small cleanups/tweaks that don't really deserve
their own patch.

- Prefer comprehensions to map() and filter(), since they're faster
- replace unused variables with _
- Use 4 spaces of indent
- drop semicolons from the end of lines
- Don't use parens around if conditions
- don't put spaces around brackets
- don't import modules as caps (ET -> et)
- Use docstrings instead of comments

v2: - Replace comprehensions with multiplication

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