clk: qcom: alpha: do not configure PLL during probe if already enabled

There are different configurations of display panels which may
or may not enable display PLLs before the display driver probes. Due to
continuous splash use cases, we may trample over existing PLL
configuration and cause the PLL to reset while it's in use. Prevent that
scenario by skipping the PLL configuration step if it's enabled.

Change-Id: I61f69c67ca57f18cbc6b551884ea199212ed4e9d
Signed-off-by: David Dai <daidavid1@codeaurora.org>