drm/gma500: Code cleanup - style fixes

Code cleanup by following i915 constant/variable names and ordering
Code cleanup by following directions from kernel doc: Codingstyle
Code cleanup by following directions from kernel doc: DRM

Signed-off-by: Arthur Borsboom <arthurborsboom@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>