drm/i915/gvt: ensure -ve return value is handled correctly

An earlier fix changed the return type from find_bb_size however the
integer return is being assigned to a unsigned int so the -ve error
check will never be detected. Make bb_size an int to fix this.

Detected by CoverityScan CID#1456886 ("Unsigned compared against 0")

Fixes: 1e3197d6

 ("drm/i915/gvt: Refine error handling for perform_bb_shadow")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>