video: fbdev: mxc: Release the previously allocated modedb before parsing edid

When we don`t set "video=mxcfb0:dev=hdmi" in bootargs,
but connect  hdmi to imx6q, it will trigger a periodically
hdmi interrupt in a few  seconds. These repeated interruptions
will read edid and alloc modedb,  so we need to release the
modedb before the next allocation.

Fix following memory leak.

unreferenced object 0xc6a54000 (size 2048):
comm "kworker/0:0", pid 2655, jiffies 756371 (age 952.680s)
hex dump (first 32 bytes):
00 00 00 00 3b 00 00 00 90 06 00 00 1a 04 00 00 ....;...........
b5 1a 00 00 18 01 00 00 68 00 00 00 1e 00 00 00 ........h.......
backtrace:
[<00623f2e>] mxc_edid_parse_ext_blk+0x79c/0x820
[<0ef13c8f>] mxc_edid_read+0x224/0x244
[<68bdc576>] mxc_hdmi_read_edid+0x6c/0x3d0
[<a64a63e8>] hotplug_worker+0x64/0x30c
[<909541fa>] process_one_work+0x1f4/0x308
[<69f4c3c3>] worker_thread+0x2b0/0x360
[<7edd79c5>] kthread+0x144/0x150
[<18f0f29a>] ret_from_fork+0x14/0x24
[<00000000>] 0x0

Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>