msm: mdss: specify unique name for panel debugfs directory

For dual DSI board configurations with independent displays,
the debugfs directory needs to be created for both panels. Use
unique directory names for the debugfs node. After this change,
the debugfs directory will have the following structure:

For Split-DSI:
/sys/kernel/debug/mdss_panel_fb0/intf0 (assuming fb0 device node)
/sys/kernel/debug/mdss_panel_fb0/intf1 (assuming fb0 device node)

For Dual-DSI with independent displays:
/sys/kernel/debug/mdss_panel_fb0/intf0 (assuming fb0 device node)
/sys/kernel/debug/mdss_panel_fb1/intf0 (assuming fb1 device node)

For Single-DSI:
/sys/kernel/debug/mdss_panel_fb0/intf0 (assuming fb0 device node)

Change-Id: Ic98d0d662932223828c41511c51cb4a0dda42bb2
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>