drm/i915/display: Use int for entry setup frames

At least one TGL had regression when using u8 types
for entry setup frames calculation. So, let's switch
to use ints instead.

intel_psr_entry_setup_frames() function expects
to return u8 but since in case of error the error
code -ETIME is returned. This doesn't fit into u8
and hence the return value is not as expected.

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: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231113093737.358137-1-mika.kahola@intel.com