drm/i915: Stop using long platform names on clock gating functions.

No functional changes.

Our code was only a bit messy with mixed style there so
let's clean up a bit using the short codenames for the platforms.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170829052026.15038-1-rodrigo.vivi@intel.com