video: tegra-dc: get clocks from device tree

DISP1 clock may use PLLP, PLLC and PLLD as parents.
Instead of hardcoding, lets pass clock and its
parent from device tree. Default parent is PLLP.

Tested-by: Robert Eckelmann <longnoserob@gmail.com> # ASUS TF101 T20
Tested-by: Nicolas Chauvet <kwizart@gmail.com> # Paz00
Tested-by: Andreas Westman Dorcsak <hedmoo@yahoo.com> # ASUS TF T30
Tested-by: Svyatoslav Ryhel <clamor95@gmail.com> # HTC One X T30
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>