drm/i915: Initialize 'data' in intel_dsi_dcs_backlight.c

drm/i915: Initialize 'data' in intel_dsi_dcs_backlight.c

variable 'data' may be used uninitialized in this function. thus,
'function dcs_get_backlight' will return unwanted value/fail.

Thus, adding NULL initialized to 'data' variable will solve the return
failure happening.

v2: Change commit message to reflect upstream with proper message

Fixes: 90198355b83c ("drm/i915/dsi: Add DCS control for Panel PWM")
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Yetunde Adebisi <yetundex.adebisi@intel.com>
Cc: Deepak M <m.deepak@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Balasubramaniam, Hari Chand <hari.chand.balasubramaniam@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1502762746-191826-1-git-send-email-hari.chand.balasubramaniam@intel.com