da8xx-fb: adopt fb_videomode data for panel information

Adopt fb_videomode data instead of defining driver private
structure to hold panel information. This is the way standard FB
drivers maintain the panel information.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>