drm/i915/frontbuffer: Mark frontbuffer flush and invalidate with might_sleep()

Frontbuffer flush and invalidate call psr, fbc and drrs functions that use
mutexes but they can be called in atomic contexts in the fbdev path. The
point where the spinlocks are acquired is up in the call stack that is not
entirely easy to spot, so annotate with might_sleep().

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180223221520.18464-2-dhinakaran.pandiyan@intel.com