drm/msm/dp: add input validation for bpc function

Add input validation to ensure that there is a valid EDID
buffer before attempting to retrieve the panel bpc value.

CRs-Fixed: 2057884
Change-Id: I43a157265dba263908e75bc2b858f522f3302edd
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>