drm/i915: use true,false for bool variable in i915_debugfs.c

Fixes coccicheck warning:

drivers/gpu/drm/i915/i915_debugfs.c:3078:4-36: WARNING: Assignment of 0/1 to bool variable
drivers/gpu/drm/i915/i915_debugfs.c:3078:4-36: WARNING: Assignment of 0/1 to bool variable
drivers/gpu/drm/i915/i915_debugfs.c:3080:4-36: WARNING: Assignment of 0/1 to bool variable
drivers/gpu/drm/i915/i915_debugfs.c:3080:4-36: WARNING: Assignment of 0/1 to bool variable

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ma Feng <mafeng.ma@huawei.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1578013959-31486-2-git-send-email-mafeng.ma@huawei.com