Allow to disable Raspberry Pi fb_copyarea acceleration via xorg.conf

Now acceleration is only used in the case if the AccelMethod option
is not set (so that it is assumed to be a default choice) or when
it is explicitly set to "COPYAREA". Any other value (for example
"CPU") disables acceleration.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>