video: da8xx-fb: store clk rate even if !CPUFREQ

store lcd clk rate always, i.e. irrespective of whether CPUFREQ is
enabled or not. This can be used to get clk rate directly instead of
enquiring with clock framework with clk handle every time.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Darren Etheridge <detheridge@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>