acpi_video: try our best to work on systems without non-essential methods

Only _BCL and _BCM methods seem to be essential to the driver's
operation. If _BQC is missing then we can assume that the current
brightness is whatever we set by the last _BCM invocation. If _DCS or
_DGS is missing the we can make assumptions as well.

The change is based on a patch suggested by Anthony Jenkins
<Scoobi_doo@yahoo.com> in PR 207086.

PR: 207086
Submitted by: Anthony Jenkins <Scoobi_doo@yahoo.com (earlier version)
Reviewed by: manu
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D24653