nine: return GetAvailableTextureMem in bytes as expected

PIPE_CAP_VIDEO_MEMORY returns the amount of video memory in megabytes,
so need to converted it to bytes.

Fixed Warframe memory detection.

Tested-by: Yaroslav Andrusyak <pontostoy@gmail.com>
Reviewed-by: Axel Davy <axel.davy@ens.fr>
Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>