drm/i915/dp: move eDP VBT bpp clamping code to intel_dp_compute_bpp()

Keep related things together. No functional changes.

v2: Fix a typo in patch subject, fix a checkpatch alignment warning.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f24d44547a586a0e342f24e69ab4d576a2474891.1524730974.git.jani.nikula@intel.com