drm/i915/display: Rename update_crtcs() to commit_modeset_enables()

This patch has no functional changes. This just renames the update_crtcs()
hooks to commit_modeset_enables() to match the drm_atomic helper naming
conventions.

v2:
* Rebase on drm-tip

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>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190827221735.29351-2-manasi.d.navare@intel.com