matroxfb: fix size of memcpy

commit 59921b239056fb6389a865083284e00ce0518db6 upstream.

hw->DACreg has a size of 80 bytes and MGADACbpp32 has 21. So when
memcpy copies MGADACbpp32 to hw->DACreg it copies 80 bytes but
only 21 bytes are valid.

Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>