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
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head/sys/dev/acpica/acpi_video.c