video: exynos_dp: Make dp_regs global

dp_regs variable was redundantly defined across all the functions in
the driver even though it contains just the same address. We make it
global and initialize it once using exynos_dp_set_base_addr().
>From then on, other funtions can use the address stored in the global variable.

Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>