drm/i915: Compute the HWS offsets explicitly

Simplify by using sizeof(u32) to convert from the index inside the HWSP
to the byte offset. This has the advantage of not only being shorter
(and so not upsetting checkpatch!) but that it matches use where we are
writing to byte addresses using other commands than MI_STORE_DWORD_IMM.

v2: Drop the now superfluous MI_STORE_DWORD_INDEX_SHIFT, it appears to
be a local invention so keeping it after the final use does not help to
clarify the GPU instruction.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190125120005.25191-2-chris@chris-wilson.co.uk