drm/i915/guc/slpc: Apply min softlimit correctly

The scenario being fixed here is depicted in the following sequence-

modprobe i915
echo 1 > /sys/class/drm/card0/gt/gt0/slpc_ignore_eff_freq
echo 300 > /sys/class/drm/card0/gt_min_freq_mhz (RPn)
cat /sys/class/drm/card0/gt_cur_freq_mhz --> cur == RPn as expected
echo 1 > /sys/kernel/debug/dri/0/gt0/reset --> reset
cat /sys/class/drm/card0/gt_min_freq_mhz --> cached freq is RPn
cat /sys/class/drm/card0/gt_cur_freq_mhz --> it's not RPn, but RPe!!

When SLPC reinitializes, it sets SLPC min freq to efficient frequency.
Even if we disable efficient freq post that, we should restore the cached
min freq (via H2G) for it to take effect.

v2: Clarify commit message (Ashutosh)

Fixes: 95ccf312

 ("drm/i915/guc/slpc: Allow SLPC to use efficient frequency")
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230621014257.1769564-1-vinay.belgaumkar@intel.com