drm/i915: Make encoder cloning more flexible

Currently we allow encoders to indicate whether they can be part of a
cloned set with just one flag. That's not flexible enough to describe
the actual hardware capabilities. Instead make it a bitmask of encoder
types with which the current encoder can be cloned.

For now we set the bitmask to allow DVO+DVO and DVO+VGA, which should
match what the old boolean flag allowed. We will add some more cloning
options in the future.

Note that this patch also removes the encoder.possible_clones setting
from encoder setup code - we compute this dynamically.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Add Ville's explanation why removing the encoder
possible_clones is save.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>