video: fbdev: pxa3xx_gcu: prepare the clocks

The clocks need to be prepared before being enabled. Without it a
warning appears in the drivers probe path :

WARNING: CPU: 0 PID: 1 at drivers/clk/clk.c:707 clk_core_enable+0x84/0xa0()
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.2.0-rc3-cm-x300+ #804
Hardware name: CM-X300 module
[<c000ed50>] (unwind_backtrace) from [<c000ce08>] (show_stack+0x10/0x14)
[<c000ce08>] (show_stack) from [<c0017eb4>] (warn_slowpath_common+0x7c/0xb4)
[<c0017eb4>] (warn_slowpath_common) from [<c0017f88>] (warn_slowpath_null+0x1c/0x24)
[<c0017f88>] (warn_slowpath_null) from [<c02d30dc>] (clk_core_enable+0x84/0xa0)
[<c02d30dc>] (clk_core_enable) from [<c02d3118>] (clk_enable+0x20/0x34)
[<c02d3118>] (clk_enable) from [<c0200dfc>] (pxa3xx_gcu_probe+0x148/0x338)
[<c0200dfc>] (pxa3xx_gcu_probe) from
[<c022eccc>] (platform_drv_probe+0x30/0x94)

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>