drm/radeon: block the keypress on ATIF events

The AMD ACPI interface may use ACPI_VIDEO_NOTIFY_PROBE to signal SBIOS
requests; block the keypress in this case since the user did not
actually press the mode switch key.

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