drm/i915: Remove KMS Kconfig option

Since we only support modesetting by default (disabling modesetting on
the command line prevents i915.ko from loading), having a parameter to
disable modesstting by default is superfluous, i.e. saying
CONFIG_DRM_I915_KMS=n is equivalent to CONFIG_DRM_I915=n.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Veter <daniel.vetter@ffwll.ch>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>