Prevent users disabling EXA UTS and DFS on radeon

[Stephen Kitt]

Hi,

The following two patches, respectively against xserver and
xf86-video-ati, allow drivers to instruct the exa module to ignore
configuration directives disabling UploadToScreen and/or
DownloadFromScreen. This may seem overkill given that these are
supposed to be debug directives, but since the driver knows it won't
work without them (as hinted by exa(4) for DownloadFromScreen), it
seems a shame not to let the exa module know about it.

Regards,

Stephen