drm/i915/display: Use int type for entry_setup_frames

entry_setup_frames variable is defined as u8. However, the
function call intel_psr_entry_setup_frames() can return
negative error code. There is a type mismatch here, so let's
switch to use int here as well.

Fixes: 2b981d57 ("drm/i915/display: Support PSR entry VSC packet to be transmitted one frame earlier")

Signed-off-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116090512.480373-1-mika.kahola@intel.com