drm: hdlcd: Add information about the underlying framebuffers in debugfs

drm_fb_cma code has a nice helper function to display in the debugfs
information about the underlying framebuffers used by HDLCD:

$ cat /sys/kernel/debug/dri/0/fb
fb: 1920x1200@XR24
   0: offset=0 pitch=7680, obj:  0 ( 2) 001011ba 0x00000000fc300000 ffffff800a27c000 9338880
fb: 1920x1200@XR24
   0: offset=0 pitch=7680, obj:  0 ( 2) 001008ca 0x00000000fba00000 ffffff8009987000 9338880
fb: 1920x1200@XR24
   0: offset=0 pitch=7680, obj:  0 ( 1) 00100000 0x00000000fb100000 ffffff8008fdc000 9216000

Add the entry in HDLCD's debugfs node.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>