drm/i915/dsi: call vbt_panel_get_modes directly instead of via drm_panel

Commit 18a00095 ("drm/i915/dsi: Make intel_dsi_enable/disable
directly exec VBT sequences") started calling the VBT sequence functions
directly instead of using the drm_panel hooks. Remove the last drm_panel
hook by calling vbt_panel_get_modes() directly. No functional changes.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/63d0d41f29583507f5968b42b5f52e6574a1f245.1488810382.git.jani.nikula@intel.com