drm/i915: Use subslice stride to set subslices for a given slice

Add a subslice stride calculation when setting subslices. This
aligns more closely with the userspace expectation of the subslice
mask structure.

v2: Use local variable for subslice_mask on HSW and
    clean up a few other subslice_mask local variable
    changes
v3: Add GEM_BUG_ON for ss_stride to prevent array overflow (Chris)
    Split main set function and refactors in intel_device_info.c
    into separate patches (Chris)
v4: Reduce ss_stride size check when setting subslices per slice
    based on actual expected max stride (Chris)
    Move that GEM_BUG_ON check for the ss_stride out to the patch
    which adds the ss_stride
v5: Use memcpy instead of looping through each stride index

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823160307.180813-8-stuart.summers@intel.com