drm/i915: Don't use the intel_ prefix for gen-specific data

We now prefix our functions/enums/data with the first platform it has
been introduced. Do that for the primary plane formats.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: s/gen2/i8xx/ and s/gen4/i965/ ...]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>