drm/i915: Store cpu_transcoder_mask in device info

We have a bunch of code that would like to know which
CPU transcoders are actually present in the hardware. Rather than
use various ad-hoc methods let's just include a full bitmask in
the device info, alongside pipe_mask.

v2: Rebase

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200318170235.15176-1-ville.syrjala@linux.intel.com


Reviewed-by: José Roberto de Souza <jose.souza@intel.com>