drm/i915: Initialize the mbus_offset to fix Klockwork issue

Static analysis identified an issue in skl_crtc_allocate_ddb where
mbus_offset may be used uninitialized.
This patch fixes it.

Fixes: 835c176c

 ("drm/i915: Introduce MBUS relative dbuf offsets")
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210603215338.13804-1-manasi.d.navare@intel.com