drm/i915/rkl: RKL only uses PHY_MISC for PHY's A and B

Since the number of platforms with this restriction are growing, let's
separate out the platform logic into a has_phy_misc() function.

Bspec: 50107
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200504225227.464666-11-matthew.d.roper@intel.com