drm/i915/rps: Freq caps for MTL

For MTL, when reading from HW, RP0, RP1 (actuall RPe) and RPn freq use an
entirely different set of registers with different fields, bitwidths and
units.

v2: Move MTL check into a separate function (Jani)

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Badal Nilawar <badal.nilawar@intel.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220910143844.1755324-4-ashutosh.dixit@intel.com