drm/i915/crt: Remove ->mode_set callback

We only set a few bits in the ADPA register, which we then read back
in the enable/disable hooks. So we can just move that bit of state
computation code to the place where we need it since setting these
bits without enabling the CRT encoder has no effects.

The only exceptions are the hotplug bits since they affect the hotplug
detection logic, but we already set those in the ->reset function and
then never touch them.

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