drm/i915: Use POSTING_READ in lcpll code

If we don't use the return value of a mmio read our coding style is to
use the POSTING_READ macro. This avoids cluttering the mmio traces.

While at it add the missing posting read in the lcpll enable function
that Paulo spotted.

v2: Drop the _NOTRACE changes, tracing such wait_for loops in the modeset
code might actually be rather useful!

Cc: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>