drm/i915: Fix pipe config mismatch for bpp, output format

Read back the pixel fomrat register and get the bpp.

v2: Read the PIPE_MISC register (Jani).

Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1556809862-31203-3-git-send-email-vandita.kulkarni@intel.com