drm/i915: Warn if a pipe is enabled with a bogus port

If TRANS_DDI_FUNC_CTL has been wrongly programmed with an incorrect
port, we are currently trying to read PORT_CLK_SEL(port) with an
uninitialized value.

Handle that case by returning PORT_CLK_SEL_NONE and warning about it.

v2: Move the warning inside intel_ddi_get_crtc_pll (Paulo Zanoni)

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