etnaviv: fix 16bpp clears

etnaviv: fix 16bpp clears

util_pack_color may leave undefined values in the upper half of the packed
integer. As our hardware needs the upper 16 bits to mirror the lower 16bits,
this breaks clears of those formats if the undefined values aren't masked off.

I've only observed the issue with R5G6B5_UNORM surfaces, other 16bpp
formats seem to work fine.

Fixes: d6aa2ba2b2 (etnaviv: replace translate_clear_color with util_pack_color)
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>