drm/i915: don't assert_pch_pll_enabled on lpt_enable_pch_transcoder

These asserts are specific to IBX/CPT/PPT. Inside the assert_pch_pll
function we even "return" in case we detect LPT, but I prefer to just
not call it. In the future we might rename to something like
ibx_assert_pch_pll.

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