drm/i915: inline skl_copy_ddb_for_pipe() to its only caller

While things may have been different before, right now the function is
very simple and has a single caller. IMHO any possible benefits from
an abstraction here are gone and not worth the price of the current
indirection while reading the code.

Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180607230700.28359-1-paulo.r.zanoni@intel.com