drm/i915/cdclk: turn around i915_drv.h and intel_cdclk.h dependency

intel_cdclk.h only needs i915_drv.h for struct intel_cdclk_config. Move
the definition to intel_cdclk.h and turn the includes around to avoid
including i915_drv.h from other headers.

The intel cdclk state macros in intel_cdclk.h still reference struct
drm_i915_private, but as macros they don't strictly require the
definition until they are used.

v2: Expand on the commit message wrt cdclk state macros

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211213114106.296017-1-jani.nikula@intel.com