drm/msm/dp: update mode validation

Use the maximum pixel clock supported on the platform as the
clock rate to validate display modes. Furthermore, once a mode
has been validated and selected, update the bits per pixel for
that mode to ensure that the mode does not exceed the maximum
allowable throughput for the chosen link configuration.

CRs-Fixed: 2119709
Change-Id: I018d4ec23436d6fac9abf9472126e0b1b70037df
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>