dri/i9*5: correctly calculate the amount of system memory

The variable name states megabytes, while we calculate the amount in
kilobytes. Correct this by dividing with the correct amount.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: "10.0 10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>