drm/i915: sseu: Convert slice count field to mask

In an upcoming patch we'll need the actual mask of slices in addition to
their count, so replace the count field with a mask.

v2:
- Use hweight8() on u8 typed vars instead of hweight32(). (Ben)

Reviewed-by: Robert Bragg <robert@sixbynine.org> (v1)
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com> (v1)
Tested-by: Ben Widawsky <benjamin.widawsky@intel.com> (v1)
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1472659987-10417-5-git-send-email-imre.deak@intel.com