drm/i915: Small joiner RAM buffer size is platform-specific

According to the bspec, GLK/CNL have a smaller small joiner RAM buffer
than ICL+.  This feels like something that could easily change again on
future platforms, so let's just add a function to return the proper
per-platform buffer size.  That may also slightly simplify the upcoming
bigjoiner enabling.

Since we have to change intel_dp_dsc_get_output_bpp()'s signature to
pass the dev_priv down for the platform check, let's take the
opportunity to also make that function static since it isn't used
outside the intel_dp file.

v2: Minor rebase on top of Maarten's changes.

Bspec: 20388
Bspec: 49259
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190925234542.24289-1-matthew.d.roper@intel.com