drm/i915/pxp: un-inline intel_pxp_is_enabled()

In the interest of reducing include dependencies, un-inline
intel_pxp_is_enabled().

v2: Fix build for CONFIG_DRM_I915_PXP=n

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/36bbb3708f3b1f84f0718afff94212dde93cb479.1639142167.git.jani.nikula@intel.com