drm/i915: Remove (pipe == crtc->index) assumption

we can't have (pipe == crtc->index) assumption in
driver in order to support 3 non-contiguous
display pipe system.

FIXME: Remove the WARN_ON(drm_crtc_index(&crtc->base) != crtc->pipe)
when we will fix all such assumption.

changes since RFC:
- Added again removed (pipe == crtc->index) WARN_ON.
- Pass drm_crtc_index instead of intel pipe in order to
  call drm_handle_vblank().
v2:
- Used drm_crtc_handle_vblank()/drm_crtc_wait_one_vblank()
  instead of drm_handle_vblank/drm_wait_one_vblank(). [Jani]
- Introduced intel_handle_vblank() helper to avoid sprinkle
  of intel_crtc across irq_handlers. [Ville]
v3:
- Moved intel_handle_vblank() from header to i915_irq.c. [Ville]

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224124004.26712-3-anshuman.gupta@intel.com