base: Split out the pixel class in framebuffer.(cc|hh).

These are really two separate things. Also, while it's realitively
straightforward to write a unit test for the pixel conversion code, the
framebuffer object is serializable and brings in more dependencies.

Change-Id: If954caeb0bfedb1002cfb1a7a115a00c90d56d19
Reviewed-on: https://gem5-review.googlesource.com/6341
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>