drm/msm: export hangcheck_period in debugfs

While keeping the previous default value for hangcheck period,
we allow now the possibility of configuring its value via
debugfs.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Link: https://lore.kernel.org/r/20210607104441.184700-1-siglesias@igalia.com
Signed-off-by: Rob Clark <robdclark@chromium.org>