swrast: Fix handling of MESA_FORMAT_L8A8_SRGB for big-endian

swrast: Fix handling of MESA_FORMAT_L8A8_SRGB for big-endian

Luminance is the least-significant byte of the uint16, rather than the
lowest byte in memory.  Other parts of mesa already handle this correctly
for big-endian, and swrast already handles other MESA_FORMAT_x8y8 formats
correctly.  This case was just an odd-one-out.

Signed-off-by: Richard Sandiford <rsandifo@linux.vnet.ibm.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>