drm/i915: Don't query PCODE RC6VIDS on platforms not supporting it

drm/i915: Don't query PCODE RC6VIDS on platforms not supporting it

On BXT/GLK GEN6_PCODE_READ_RC6VIDS fails with
MAILBOX_P24C_CC_ILLEGAL_CMD, so don't try to do the query on these
platforms. Do it only on SNB, IVB and HSW, where we use this command
anyway for RC6 enabling.

Based on my tests the command also succeeds on all LLC platforms, but
it's not clear if it's really supported on those (it returns 0 aka 245mv
for all RC6 states everywhere except on SNB). BSpec lists the command as
supported on SKL+ (see P24C_PCODE_MAILBOX_INTERFACE) but that's clearly
incorrect, since on SKL/KBL the same command ID is used for
SKL_PCODE_LOAD_HDCP_KEYS. Since the command fails on BXT/GLK, the BSpec
command list is also incorrect for those platforms (see
P_CR_P24C_PCODE_MAILBOX_INTERFACE_0_2_0_GTTMMADR).

I filed a request to update that info in Bspec, but for now let's
assume a minimal set of platforms where the command is supported.

References: https://bugs.freedesktop.org/show_bug.cgi?id=103337
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208174102.10240-1-imre.deak@intel.com