mesa: clear existing swizzle info before bitwise-OR

mesa: clear existing swizzle info before bitwise-OR

This patch fixes a bug in big-endian treatment, where the previous
swizzle info wasn't cleared before a new swizzle info was inserted into
the format field using a bitwise-OR operation.

v2: use MESA_ARRAY_FORMAT_SWIZZLE_*_MASK instead of numeric constants
v3: align according to coding style

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
CC: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>