drm/i915/gt: Add media freq factor to per-gt sysfs

Expose new sysfs to program and retrieve media freq factor. Factor values
of 0 (dynamic), 0.5 and 1.0 are supported via a u8.8 fixed point
representation (corresponding to integer values of 0, 128 and 256
respectively).

Media freq factor is converted to media_ratio_mode for GuC. It is
programmed into GuC using H2G SLPC interface. It is retrieved from GuC
through a register read. A cached media_ratio_mode is maintained to
preserve set values across GuC resets.

This patch adds the following sysfs files to gt/gtN sysfs:
* media_freq_factor
* media_freq_factor.scale

v2: Minor wording change in drm_warn (Tvrtko)

Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Dale B Stimson <dale.b.stimson@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7ad7578335d8af9cba047b4bcf33d1887453d2e1.1653484574.git.ashutosh.dixit@intel.com