drm/i915: Move intel_init_clock_gating() to i915_gem_init()

Despite its name intel_init_clock_gating applies both display clock gating
workarounds; GT mmio workarounds and the occasional GT power context
workaround. Worse, sometimes it includes a context register workaround
which we need to apply before we record the default HW state for all
contexts.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110142634.10551-4-chris@chris-wilson.co.uk