drm/i915/irq: reduce inlines to reduce header dependencies

Presumably if the compiler is smart, it does not generate an extra
function call to the update functions that are now static.

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/594f628740717cda5ef407a26ea03129c22ddc12.1629281426.git.jani.nikula@intel.com