drm/i915: Add has_master_unit_irq flag

Add flag to differentiate platforms with and without the master
IRQ control bit.

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200713182321.12390-1-lucas.demarchi@intel.com