ACPI video: allow events handlers to veto the keypress

The standard video events may be overloaded for device specific
purposes. For example AMD ACPI interface overloads
ACPI_VIDEO_NOTIFY_PROBE (0x81) to signal AMD-specific events. In such
cases we don't want to send the keypress (KEY_SWITCHVIDEOMODE) to the
userspace because the user did not press the mode switch key (the
spurious keypress confuses the DE which usually changes the
display configuration and messes up a dual-screen setup).
This patch gives the handlers the chance to examine the event and
block the keypress if the event is device specific.
v2: refactor as suggested by Zhang Rui <rui.zhang@intel.com>

Signed-off-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>