disp: pll: remove error log if supply entries are absent

PLL supply entry is no longer a mandatory property. Do not
report error if this entry is not present.

CRs-Fixed: 2588602
Change-Id: I1d8ff9e7e56f419c6a8984b69f64d61084e6c0f6
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>