drm/i915/display: Move the commit_tail() disable sequence to separate function

Create a new function intel_commit_modeset_disables() consistent
with the naming in drm atomic helpers and similar to the enable function.
This helps better organize the disable sequence in atomic_commit_tail()

No functional change

v4:
* Do not create a function pointer, just a function (Maarten)
v3:
* Rebase (Manasi)
v2:
* Create a helper for old_crtc_state disables (Lucas)

Suggested-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190828224701.422-1-manasi.d.navare@intel.com