drm/i915/query: fix subslice length

We dump the info as an array of u8, so we want to know the length
in number of bytes. Current code is still safe because the
variable we use BITS_PER_TYPE on is a u8.

Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181109004013.34394-1-daniele.ceraolospurio@intel.com