i915: Reject CCS modifiers for pipe C on Geminilake

Current code advertises (on the modifiers blob property) support for CCS
modifier for pipe C on GLK, only to reject it later when validating the
request before the atomic commit.

This fixes the tests igt@kms_ccs@pipe-c-*, which should skip on GLK for
pipe C (see bug 104096).

A relevant discussion is archived at:

https://lists.freedesktop.org/archives/intel-gfx/2017-December/150646.html

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104096
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Cc: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171220002410.5604-1-krisman@collabora.co.uk
(cherry picked from commit f0cbd8bd

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>