drm: Fix HDCP downstream dev count read

In both HDMI and DP, device count is represented by 6:0 bits of a
register(BInfo/Bstatus)

So macro for bitmasking the device_count is fixed(0x3F->0x7F).

v3:
  Retained the Rb-ed.
v4:
  %s/drm\/i915/drm [rodrigo]
v5:
  Added "Fixes:" and HDCP keyword in subject [Rodrigo, Sean Paul]

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Fixes: 495eb7f8 drm: Add some HDCP related #defines
cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1522929802-22850-1-git-send-email-ramalingam.c@intel.com