video: da8xx-fb: clk_get on connection id fck

do clk_get on connection id "fck" to support OMAP based
platforms having multiple clocks for module. Without this
driver change clk_get fails on am335x.

This patch is based on the discussion in community
http://marc.info/?l=linux-kernel&m=135166018907827&w=2

Signed-off-by: Manjunathappa <prakash.pm@ti.com>
Cc: Vaibhav Hiremath <hvaibhav@ti.com>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>