Fix esphome/issues#947 - RGBW(W) white brightness

[pauln]

Description:

Updates the as_rgbw() and as_rgbww() calculations to factor in the brightness as well as the white level, so that the white channels dim as expected when the brightness is changed.

Related issue (if applicable): fixes #947

Pull request in esphome-docs with documentation (if applicable): N/A

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[glmnet]

LGTM

[jjross]

First, thanks for all the good work.

When do we think this will be added to the mainline code? I'm affected by the bug.