drm/i915: explicitly set up PIPECONF (and gamma table) on haswell

Again we don't really support different settings, so don't let the
BIOS sneak stuff through.

Since the motivation for this patch series is to ensure we have the
correct gamma table mode selected also add the required write to the
GAMMA_MODE register to select the 8bit legacy table.

And since I find lowercase letters in #defines offensive, also
bikeshed those.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>