drm/amd/display: handle nulled pipe context in DCE110's set_drr()

As set_drr() is called from IRQ context, it can happen that the
pipe context has been nulled by dc_state_destruct().

Apply the same protection here that is already present for
dcn35_set_drr() and dcn10_set_drr(). I.e. fetch the tg pointer
first (to avoid a race with dc_state_destruct()), and then
check the local copy before using it.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3142


Fixes: 06ad7e16 ("drm/amd/display: Destroy DC context while keeping DML and DML2")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>