drm/i915/dp: fix integer overflow in 128b/132b data rate calculation

The intermediate value 1000000 * 10 * 9671 overflows 32 bits, so force
promotion to a bigger type.

From the logs:

[drm:intel_dp_compute_config [i915]] DP link rate required 3657063 available -580783288

v2: Use mul_u32_u32() (Ville)

Fixes: 48efd014 ("drm/i915/dp: add max data rate calculation for UHBR rates")
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211026093407.11381-1-jani.nikula@intel.com