ARM: DRA7: emif: Check for enable bits before updating leveling output

Read and write leveling can be enabled independently. Check for these
enable bits before updating the read and write leveling output values.
This will allow to use the combination of software and hardware leveling.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>