drm/i915: Add WARN_ON in intel_get_crtc_for_pipe()

Add a WARN_ON for a disabled pipe in pipe_mask at
intel_get_crtc_for_pipe() function.

v2:
- Use drm_WARN_ON instead of WARN_ON.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224124004.26712-7-anshuman.gupta@intel.com