drm/i915: rename BROADWATER and CRESTLINE to I965G and I965GM, respectively

Add more consistency to our naming. Pineview remains the outlier. Keep
using code names for gen5+.

v2: rebased

Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1481105584-23033-1-git-send-email-jani.nikula@intel.com