drm/i915: split out i915_freq.[ch]

drm/i915: split out i915_freq.[ch]

The i915 core only needs three rather specific functions from
soc/intel_dram.[ch]: i9xx_fsb_freq(), ilk_fsb_freq(), and
ilk_mem_freq(). Add new i915_freq.[ch] and duplicate those functions for
i915 to reduce the dependency on soc/ code.

Wile duplication in general is bad, here it's a tradeoff to simplify the
i915, xe and display interactions.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/7bac1b194afdc20cd45e625a0a32fcbcd0b1136e.1763578288.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>