ARM: OMAP2+: Simplify code for clkdm_clock_enable and disable

We can make clkdm_clk_enable() usable for clkdm_hwmod_enable() by
dropping the unused clock check, and drop _clkdm_clk_hwmod_enable().

And we can make clkdm_hwmod_disable() call clkdm_hwmod_disable() and drop
the duplicate code in clkdm_hwmod_disable().

Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>