drm/i915: move memory frequency detection to intel_dram.c

The memory frequency detection is a bit spread out here and
there. Consolidate to intel_dram.c.

v2:
- Remove inaccurate comment (Ville)
- Call detect_mem_freq() unconditionally (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8a862eeca8b42a98e04b3c52637851d33531abb6.1676317696.git.jani.nikula@intel.com