drm/i915: make intel_dp_compute_bpp static

Fix sparse warning:

drivers/gpu/drm/i915/intel_dp.c:1527:5: warning: symbol
'intel_dp_compute_bpp' was not declared. Should it be static?

Fixes: f9bb705e ("drm/i915: Update bits per component for display info")
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Reviewed-by: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473946137-1931-1-git-send-email-jani.nikula@intel.com