disp: msm: dp: validate edid before dereferencing

Currently, when using custom edid from debugfs, the
extensions data inside the edid block is not validated
before dereferencing the extension block.

The fix adds a edid validation function to validate
any custom edids before accessing any members in the
edid block.

Change-Id: I8a2cc45477416a8f8c4cff882bd53d14012e29f4
Signed-off-by: Sudarsan Ramesh <sudarame@codeaurora.org>