drm/i915: handle DDI-related assertions

drm/i915: handle DDI-related assertions

Prevent bogus asserts on DDI-related paths.

Longer explanation from Eugeni by mail:

"For the asserts there are 3 paths where we hit them:
- in assert_fdi_tx (we don't have the FDI_TX_CTL anymore, backup plan
  DDI_FUNC_CTL is used instead)
- in assert_fdi_tx_pll_enabled (we have the combination of iCLKIP and
  DDI_FUNC_CTL, plus PORT_CLK_SEL and PIPE_CLK_SEL now to make things
  work). We could use an assert here indeed - if we configure port to
  use one clock, and pipe to use another, everything hangs. Right now,
  we configure all of them in one place only; but yes, when DP code
  lands it will get more funky.
- and in ironlake_fdi_pll_enable. I reuse part of this function (to
  configure the TU sizes), but as in the 1st case, FDI_TX_CTL is gone
  so I just ignore it here."

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
[danvet: Pasted Eugeni's explanation into the commit message.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>