drm/i915/display: Do not reset display when there is none

Display is always disabled and enabled when resetting any engine, but if
there is no display it should not do anything with display and only
reset the needed engines.

Cc: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201106225531.920641-3-lucas.demarchi@intel.com