drm/imagination: Fix IS_ERR() vs NULL bug in pvr_request_firmware()

The pvr_build_firmware_filename() function returns NULL on error.  It
doesn't return error pointers.

Fixes: f99f5f3e ("drm/imagination: Add GPU ID parsing and firmware loading")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/384288de-a779-46c7-869d-b3c63462e12b@moroto.mountain