drm/i915/display: Add own counter for Panel Replay vblank workaround

We are about to change meaning of vblank_enabled to fix Panel Replay vblank
workaround. For sake of clarity we need to rename it. Vblank_enabled is
used for i915gm/i945gm vblank irq workaround as well -> instead of rename
add new counter named as vblank_wa_pipes.

v2:
  - s/vblank_wa_pipes/vblank_wa_num_pipes/
  - use int as a type for the counter

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241009134225.1322820-1-jouni.hogander@intel.com