drm/i915/dmc: Add intel_dmc_has_payload() helper

We check for dmc_payload being there at various points in the driver.
Replace it with the helper.

v2: rebased.
v3: Move intel_dmc to intel_dmc.h in another patch (Lucas)
v4: Remove headers not needed from intel_dmc.h

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210526220256.4097-3-anusha.srivatsa@intel.com