ENGR00180497 FB: Fix a bug: 'fb_set_par error, -22' when video playback on lvds

ENGR00180497 FB: Fix a bug: 'fb_set_par error, -22' when video playback on lvds

Change bg_id/fg_id variable to char array variable, and
avoid change the constant string in .rodata section.

[MX6Q SMD]fb: 'fb_set_par error, -22' prompted when mplay a video to lvds
/mnt/nfs/util/mplayer /mnt/nfs/test_stream/video/Mpeg4_SP1_1280x720_30fps.mp4
Before video playback finish, fb error message shows:

mxc_sdc_fb mxc_sdc_fb.0: setup error, dispdrv:ldb.
detected fb_set_par error, error code: -22
Can't reset original fb_var_screeninfo: Invalid argument
Exiting... (End of file)

Signed-off-by: Wayne Zou <b36644@freescale.com>