The first piece, intel_ddi_pll_select, finds a PLL and assigns it to
the CRTC, but doesn't write any register. It can also fail in case it
doesn't find a PLL.

The second piece, intel_ddi_pll_enable, uses the information stored by
intel_ddi_pll_select to actually enable the PLL by writing to its
register. This function can't fail. We also have some refcount sanity
checks here.

The idea is that one day we'll remove all the functions that touch
registers from haswell_crtc_mode_set to haswell_crtc_enable, so we'll
call intel_ddi_pll_select at haswell_crtc_mode_set and then call
intel_ddi_pll_enable at haswell_crtc_enable. Since I'm already
touching this code, let's take care of this particular split today.

v2: - Clock on the debug message is in KHz
    - Add missing POSTING_READ
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NDamien Lespiau <damien.lespiau@intel.com>
[danvet: Bikeshed comments.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>