drm/gma500: add a missed gma_power_end in error path

oaktrail_lvds_mode_set() misses a gma_power_end() in an error path.
Add the call to fix it.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209085747.16057-1-hslester96@gmail.com