drm/i915: Add locking to pll updates, v3.

drm/i915: Add locking to pll updates, v3.

With async modesets this is no longer protected with connection_mutex,
so ensure that each pll has its own lock. The pll configuration state
is still protected; it's only the pll updates that need locking against
concurrency.

Changes since v1:
- Rebased.
- Fix locking to protect all accesses. (Durgadoss)
Changes since v2:
- Make the dpll_lock global to protect concurrent updates to the
  same register, for example DPLL_CTRL1 on skl. (Ander)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/56F29F50.1090708@linux.intel.com
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>