drm/i915/dp: Simplify input BPP checks in intel_dp_dsc_compute_pipe_bpp()

Drop the extra local variables and simplify the conditions. We don't
have to try to special case the loop condition and break in the validity
checks.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5559b14d6af4e001677f23454d6bd8b3606b3d7a.1738327620.git.jani.nikula@intel.com