video: tegra-dc: request timings from panel driver first

video: tegra-dc: request timings from panel driver first

Check if panel driver has display timings and get those.
If panel driver does not pass timing, try to find timing
under rgb node for backwards compatibility.

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> # LG P895 T30
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>