drm/gma500: Calculate clock in one function instead of three identical

i9xx_clock() and i8xx_clock() did the same calc and psb_intel_clock() just
called i9xx_clock() so just move it all into psb_intel_clock().

The same calculation is duplicated in cdv_intel_display.c as well so maybe we
can share it later on.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>