drm/i915: Extract i915gm_irq_cstate_wa_{disable,enable}()

drm/i915: Extract i915gm_irq_cstate_wa_{disable,enable}()

Extract the i915gm/i945gm vblank irq C-state workaround to
separate functions. We'll need to reuse these in order to
guarantee timely CRC interrupt delivery as well.

The irq.vblank_enabled count is currently protected by the
drm vblank locks, so let's assert that the innermost of those
is held, in anticipation of other callers.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241001195803.3371-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>