drm/i915: Enable non-contiguous pipe fusing

As we have already enabled supported infrastructure for
non-contiguous pipe fusing in driver, we don't require non-contiguous
pipe_mask check anymore.
It is an unlike condition, it make sense to remove this condition.

changes since RFC:
- using intel_pipe_mask_is_valid() function to check integrity of
  pipe_mask. [Ville]
v2:
- simplify condition in intel_pipe_mask_is_valid(). [Ville]
v3:
- removed non-contiguous pipe fusing check. [Lucas]

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200318094448.26239-1-anshuman.gupta@intel.com