drm/i915/dsi: drop unused but set variable vbp

Prepare for re-enabling -Wunused-but-set-variable.

The vbp is not used for anything in the readout, as we get
e.g. crtc_vtotal from BXT_MIPI_TRANS_VTOTAL.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/27efd245aa75226adcac01eff7b21781970f2736.1685119007.git.jani.nikula@intel.com