mode fixes for tda998x.

* 'tda998x-fixes' of git://ftp.arm.linux.org.uk/~rmk/linux-cubox:
  drm/i2c: tda998x: add some basic mode validation
  drm/i2c: tda998x: faster polling for edid
  drm/i2c: tda998x: move drm_i2c_encoder_destroy call