msm: camera: common: Fix enable/disable for dsp clock source

Currently, if dsp clock source is not found in device tree, still
we are trying to enable/disable clock source. Check for availability
of that optional clock before toggling it.

CRs-Fixed: 2762106
Change-Id: I2a4467f5383343bdf1e6607712270f574c76e51e
Signed-off-by: Mukund Madhusudan Atre <matre@codeaurora.org>