rk fb : fix a panic bug

rk fb : fix a panic bug
	panic bug like this:
	rk_fb_wait_for_vsync_thread may access inf->fb[0] before it is initialized
	[    1.009224] Unable to handle kernel NULL pointer dereference at virtual address 00000240
	[    1.017323] pgd = c0404000
	[    1.020035] [00000240] *pgd=00000000
	[    1.023620] Internal error: Oops: 5 [#1] PREEMPT SMP
	[    1.028586] CPU: 1    Not tainted  (3.0.36+ #217)
	[    1.033297] PC is at rk_fb_wait_for_vsync_thread+0xe8/0x144
	[    1.112136] LR is at rk_fb_wait_for_vsync_thread+0xe8/0x144
	[    1.117708] pc : [<c06243e0>]    lr : [<c06243e0>]    psr: 60000013
	[    1.117715] sp : efe59f80  ip : c04384a0  fp : 00000000
	[    1.129194] r10: 00000000  r9 : efe59f90  r8 : efe58000
	[    1.134417] r7 : efe39a98  r6 : 00000000  r5 : efe59f84  r4 : efe39a08
	[    1.140942] r3 : 00000000  r2 : 00000000  r1 : efc475a0  r0 : 00000036
	[    1.147469] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment kernel
	[    1.154777] Control: 10c5387d  Table: 6040404a  DAC: 00000015