lib: keep a list of modified connectors to reset

lib: keep a list of modified connectors to reset

Avoid calling functions in igt_reset_connectors that are not safe to use
in signal handlers by keeping a list of connectors that have been
modified, instead of enumerating all connectors.

v2: add space for a sentinel NULL value on forced_connectors and print a
    warning when the connector limit is reached (Chris Wilson)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83498
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Thomas Wood <thomas.wood@intel.com>