drm/i915: Move calculation of subslices per slice to new function

Add a new function to return the number of subslices per slice to
consolidate code usage.

v2: rebase on changes to move sseu struct to intel_sseu.h
v3: add intel_* prefix to sseu_subslices_per_slice

Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524154022.13575-4-stuart.summers@intel.com