Make minimum screen brightness not always be 0

With some drivers, a value of 0 disables the screen backlight
completely, which is almost certainly not what the user wanted. The
Battery & Brightness widget has logic to handle this and instead make
1 the minimum brightness for screens. But Powerdevil does not, and as a
result, the behaviors differ between adjusting the brightness using the
widget and using the keyboard's "brightness down" key. This is
inconsistent and weird.

This commit unifies the behaviors by adding logic to Powerdevil to match
what the widget does.

Arguably this is working around a driver bug, but insisting that all
graphics driver bugs be fixed is not practical due to their diversity
and frequent regressions. And the Battery & Brightness widget already
has such a workaround to prevent users whose graphics drivers don't
work as they should from experiencing this issue, so not doing the same
here is inconsistent.

BUG: 467910
FIXED-IN: 6.0