Add missing prepare/unprepare operations for fbi->clk,
this fixes following kernel warning:

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 1 at drivers/clk/clk.c:874 clk_core_enable+0x2c/0x1b0
  Enabling unprepared disp0_clk
  Modules linked in:
  CPU: 0 PID: 1 Comm: swapper Not tainted 4.18.0-rc8-00032-g02b43ddd4f21-dirty #25
  Hardware name: Marvell MMP2 (Device Tree Support)
  [<c010f7cc>] (unwind_backtrace) from [<c010cc6c>] (show_stack+0x10/0x14)
  [<c010cc6c>] (show_stack) from [<c011dab4>] (__warn+0xd8/0xf0)
  [<c011dab4>] (__warn) from [<c011db10>] (warn_slowpath_fmt+0x44/0x6c)
  [<c011db10>] (warn_slowpath_fmt) from [<c043898c>] (clk_core_enable+0x2c/0x1b0)
  [<c043898c>] (clk_core_enable) from [<c0439ec8>] (clk_core_enable_lock+0x18/0x2c)
  [<c0439ec8>] (clk_core_enable_lock) from [<c0436698>] (pxa168fb_probe+0x464/0x6ac)
  [<c0436698>] (pxa168fb_probe) from [<c04779a0>] (platform_drv_probe+0x48/0x94)
  [<c04779a0>] (platform_drv_probe) from [<c0475bec>] (driver_probe_device+0x328/0x470)
  [<c0475bec>] (driver_probe_device) from [<c0475de4>] (__driver_attach+0xb0/0x124)
  [<c0475de4>] (__driver_attach) from [<c0473c38>] (bus_for_each_dev+0x64/0xa0)
  [<c0473c38>] (bus_for_each_dev) from [<c0474ee0>] (bus_add_driver+0x1b8/0x230)
  [<c0474ee0>] (bus_add_driver) from [<c0476a20>] (driver_register+0xac/0xf0)
  [<c0476a20>] (driver_register) from [<c0102dd4>] (do_one_initcall+0xb8/0x1f0)
  [<c0102dd4>] (do_one_initcall) from [<c0b010a0>] (kernel_init_freeable+0x294/0x2e0)
  [<c0b010a0>] (kernel_init_freeable) from [<c07e9eb8>] (kernel_init+0x8/0x10c)
  [<c07e9eb8>] (kernel_init) from [<c01010e8>] (ret_from_fork+0x14/0x2c)
  Exception stack(0xd008bfb0 to 0xd008bff8)
  bfa0:                                     00000000 00000000 00000000 00000000
  bfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
  bfe0: 00000000 00000000 00000000 00000000 00000013 00000000
  ---[ end trace c0af40f9e2ed7cb4 ]---
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
[b.zolnierkie: enhance patch description a bit]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>