drm/i915: add wait_for_vblank argument to intel_enable_pipe

Depending on the HW gen and the connector type, the pipe won't start
running right after we call intel_enable_pipe, so that
intel_wait_for_vblank call we currently have will just sit there for
the full 50ms timeout. So this patch adds an argument that will allow
us to avoid the vblank wait in case we want. Currently all the callers
still request for the vblank wait, so the behavior should still be the
same.

We also added a POSTING_READ on the register: previously
intel_wait_for_vblank was acting as a POSTING_READ, but now if
wait_for_vblank is false we'll stkip it, so we need an explicit
POSTING_READ.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>