drm/i915: relocate some DSPCNTR reg bit definitions

Some plane B/C specific bits were left next to the unused _DSPBCNTR
macro. Move them next to the DSPCNTR() macro.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/85409fbe5073797c0dc17df43eeb25abe9ff889f.1717773890.git.jani.nikula@intel.com


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