drm/radeon: finish getting bios earlier

Return a number of bytes read in radeon_atrm_get_bios_chunk() and
properly check this value in radeon_atrm_get_bios().
If radeon_atrm_get_bios_chunk() read less bytes then were requested,
it means that it finished reading bios data.

Prior to this patch, condition in radeon_atrm_get_bios() was always
equivalent to "if (ATRM_BIOS_PAGE <= 0)", so it was always false,
thus radeon_atrm_get_bios() was trying to read past the bios data
wasting boot time.

On my lenovo ideapad u455 laptop this patch drops bios reading time
from ~5.5s to ~1.5s.

Signed-off-by: Igor Murzov <e-mail@date.by>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>