drm/i915: Move drm_vblank_init() to i915_load_modeset_init()

i915_load_modeset_init() is a more suitable place than
i915_driver_load() as vblank is part of modeset.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181108001647.11276-1-jose.souza@intel.com