drm/i915: remove some unused declarations from intel_drv.h

intel_mark_busy(), intel_mark_idle(), and skl_cdclk_get_vco() no longer
exist.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606122203.13416-2-jani.nikula@intel.com