mga_drv: fix image corruption due to memcpy reordering

The memcpy specification doesn't specify how the destination is written -
and on glibc 2.19 memcpy is doing some bytewise writes with the rep stosb
instruction. These bytewise writes confuse the hardware (because it
expects 32-bit writes) and result in image corruption.

This patch replaces memcpy with explicit writes, so that the sequential
write order and 32-bit write size is guaranteed. We use the "volatile"
attribute to make sure that compiler optimizations won't cause write
reordering. When reading the memory, we use a structure with "packed"
attribute to avoid unaligned traps on RISC machines.

Tested-by: Kevin Brace <kevinbrace@gmx.com>
Signed-off-by: Mikulas Patocka <mikulas@twibright.com>