Partially revert "drm/i915: s/mdelay/msleep/" in ilk rps code

This reverts commit 6adfb1ef.

Ironlake RPS code runs under an irqsave spinlock and hence sleeping
isn't allowed. Not a this long delay while blocking irqs isn't great
at all, but fixing the locking scheme is a lot more involved.

So just revert for now.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-by: kernel test robot <ying.huang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>