drm/i915: remove redundant checks for a null fb pointer

A prior check and return when pointer fb is null makes
subsequent null checks on fb redundant.  Remove the redundant
null checks.

Addresses-Coverity: ("Logically dead code")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210142349.333171-1-colin.king@canonical.com