drm/i915: rework reading pipe disable fuses

drm/i915: rework reading pipe disable fuses

This prepares to have possibly more than 3 pipes. I didn't want to
continue the previous approach since the check for "are the disabled
pipes the last ones" poses a combinatory explosion. We need that check
because in several places of the code we have that assumption. If that
ever becomes false in a new HW, other parts of the code would have to
change.

Now we start by considering we have info->num_pipes enabled and disable
each pipe that is marked as disabled. Then it's a simple matter of
checking if we have at least one pipe and that all the enabled ones are
the first pipes, i.e. there are no holes in the bitmask.

Cc: Jose Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625175437.14840-3-lucas.demarchi@intel.com