drm/i915: Fix up cpt pixel multiplier enable sequence

Bspec for the "DPLL HDMI multiplier" field says:

"Restriction : The DPLL must be enabled and stable before setting these bits.
These bits must be programmed after DPLL_SEL is programmed."

There is apparently no restriction on programming the DPLL_SEL
register wrt the DPLL. So let's just move that up before we enable the
pch dpll.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>