etnaviv: avoid using invalid TS

etnaviv: avoid using invalid TS

The TS is only valid after it has been initialized by a fast
clear, so it should not be taken into account when blitting
resources that haven't been cleared. Also the blit itself
invalidates the destination TS, as it's not updated and will
retain data from the previous rendering after the blit.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com>