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 <c2fce4493a16d94b58efb315151c29911fc5d347@gmail.com>
CC: "10.5 10.6" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Reviewed-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler