drm/i915: use i9xx_fsb_freq() for GT clock frequency

drm/i915: use i9xx_fsb_freq() for GT clock frequency

Reuse i9xx_fsb_freq() for GT clock frequency initialization instead of
depending on rawclk_freq.

Note: If the init order was changed, we could use i915->fsb_freq
directly. However, GT clock initialization is done in
i915_driver_mmio_probe(), but intel_dram_detect() later in
i915_driver_hw_probe(), with a dependency on intel_pcode_init().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0678d8ec9772725b47d4fa5b14e3b3a34256d5cf.1718356614.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>