ARM: OMAP: Remove unnecessary call to clk_get()

Whenever we call the function omap_dm_timer_set_source() to set the clock
source of a dmtimer we look-up the dmtimer functional clock source by
calling clk_get(). This is not necessary because on requesting a dmtimer
we look-up the functional clock source and store it in the omap_dm_timer
structure. So instead of looking up the clock again used the clock handle
that stored in the omap_dm_timer structure.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>