drm/i915/display: Rename vblank DC workaround functions and variables

We have extended using vblank DC workaround mechanism for
Wa_16025596647. Rename related functions and variables:

vblank_wa_num_pipes -> vblank_enable_count
vblank_dc_work -> vblank_notify_work
intel_display_vblank_dc_work -> intel_display_vblank_notify_work

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250414100508.1208774-14-jouni.hogander@intel.com